John D. Norton

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ORAL HISTORIES

Credit: University of Pittsburgh

Interviewed by
David Zierler
Location
Video conference
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Interview of John D. Norton by David Zierler on June 5, 2020,Niels Bohr Library & Archives, American Institute of Physics,College Park, MD USA,www.aip.org/history-programs/niels-bohr-library/oral-histories/44816

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Abstract

In this interview, David Zierler, Oral Historian for AIP, interviews John D. Norton, distinguished professor of history and philosophy of science at the University of Pittsburgh. Norton recounts his experiences as a child of Jewish refugees who emigrated to Australia after World War II. He discusses his early interest in science and his original intention to pursue a career as a scientist before he became fascinated by issues in the philosophy of science as a graduate student at the University of New South Wales, and the deep influence of Kuhn on his development as a scholar. He describes his dissertation and subsequent and sustained research interest on Einstein and general relativity. Norton describes his initial work in the Einstein Papers at Princeton where he continued to work with documents that had never been systematically analyzed before. He explains the circumstances leading to his decision to join the faculty at the University of Pittsburgh and he describes the interplay of science and philosophy in his department. Toward the end of the interview, Norton expounds on a variety of theoretical and philosophical ideas that emanate from the history of science, and from quantum mechanics specifically, and he describes the importance of being able to convey Einstein’s contributions to a broad audience.

Transcript

Zierler:

This is David Zierler, oral historian for the American Institute of Physics. It is June 5th, 2020. It is my great pleasure to be here with Professor John D. Norton. John, thank you so much for being with me today.

Norton:

My pleasure, happy to be here.

Zierler:

To start, please tell me your title and institutional affiliation.

Norton:

I'm a Distinguished Professor in the Department of History and Philosophy of Science at the University of Pittsburgh.

Zierler:

Now, let’s go right back to the beginning. Tell me about your family background. Tell me about your parents and what they did for a living and where they came from.

Norton:

My parents were European refugees. My father was a Czech Jew who fled Czechoslovakia at the beginning of World War II and made it over to London during the blitz. My mother was a rather bourgeois German Jew who lived in Berlin. Her family were bankers, and she also managed to escape at the very last minute and end up in London. There, they married. My father joined the British army. He was part of the British Expeditionary Force that went over to France and barely made it back to England after the Dunkirk evacuation. They eventually emigrated to Australia in 1948, under the Assisted Passage Plan. Australia was very, very keen to bring in white Anglo-Saxon workers, whatever they could get. It was an overtly racist immigration policy. And so they made it very cheap for English people to come. Since my parents had acquired British nationality from service in the army, they counted. For ten pounds, they got an assisted passage to Australia. My father was in the Royal Engineers in the British Army, and there he learned his only useful skills. To grow up in a shtetl in Eastern Europe, you don’t have useful skills. There, he learned engineering. More precisely, it was machining. He came to Australia and he started opening small machine shops. The earliest one that I can remember was a machine shop where you would come to him and he would build whatever machine you liked. One of the machines that he built was a Lifesavers sorting machine. The factory would put the Lifesavers into it as they came out of the manufacturing process. It would separate out the broken ones from the unbroken ones. He also made bacon smokers.

Zierler:

[laugh]

Norton:

Then he gradually moved on from there, and eventually opened a company—“Pressure Fittings”—where he made pipe fittings for oil refineries and related plant. High-pressure fittings.

Zierler:

What language did your parents speak to each other in the early years?

Norton:

They always spoke English to one another. You can just imagine what it would be like in the Blitz in London to be speaking German to each other. It’s not going to end well. So I unfortunately did not learn German at home. I wish I had.

Zierler:

Or Czech, for that matter.

Norton:

Or Czech, yes. Well, the German would have been enormously useful for my work on Einstein. I had to learn German as a school language. I did hear a fair amount of German, though, with an aunt that we would visit regularly. She was from Vienna. And so we would do the typical Viennese afternoon coffee and cake with all the flourishes, all the “servus” ritual that people from Vienna like to do—rather exaggerated rituals of politeness with linen and silver services and so on.

Zierler:

I'm curious if in 1948 your parents considered Israel as a place to settle.

Norton:

I don’t think they did. That’s not for lack of relatives in Israel. On both sides of the family, there were relatives in Israel, whom I did visit a number of times.

Zierler:

And what about the United States? Did they consider the U.S.?

Norton:

I don’t know how easy it would have been to come to the U.S. Remember, they had ten-pound assisted passages. You pay ten pounds; they put you on a boat, and they send you over to Australia. This is a country that wanted them. It wasn’t clear that the U.S. did.

Zierler:

So they didn't want to stay in England. They were looking for someplace else to go.

Norton:

Yes.

Zierler:

Why?

Norton:

I'm not entirely sure why. I think there was a sense of tiredness in the country after the enormous upheavals of the war. I think there’s something for war refugees to want to get as far away from Europe as possible. My wife’s family has a similar history. They were German, Polish, and Viennese, and they had the misfortune of fleeing eastward rather than westward, and so they were eventually interned by the Russians and spent the whole war in Russian labor camps. And then after the war, they stayed in Russian labor camps for another two years and were only able to get out because the Red Cross published a list of names of people who were essentially imprisoned. They got out, and they clearly wanted to get as far away from Europe as possible.

Zierler:

So you were born in Australia.

Norton:

I was born in Australia. The reason for Sydney for both my family and my wife’s family was that they had relatives there. So they had a local connection that would make it easier for them to come and settle in.

Zierler:

Growing up, was your family mostly secular? Were you Jewishly connected at all?

Norton:

Yes, we had Jewish connections all the way through. The Jewish community in Sydney is fairly small. It divides only into two groups, Orthodox and Liberal. We first went to an Orthodox synagogue, which was for me a rather unpleasant experience, because my parents weren’t keeping a Kosher household or doing much of anything. We’d go for Yom Kippur and Rosh Hashanah services and events like that, and take Sunday school there. I was repeatedly reminded by the rabbi of how bereft I was of any proper Jewish background. [laugh] So it was just unpleasant. And then we went to a liberal synagogue, a temple, and that was much more comfortable.

Zierler:

I'm curious, just to foreshadow with your interest in Albert Einstein, if you felt not just a scholarly interest but a cultural affinity for another German Jew.

Norton:

That wasn’t a motivation, although that certainly happened. Einstein affiliated culturally with the Jewish people, as I do as well. Although I don’t practice; I have no special religious interest.

Zierler:

Did you go to public school throughout, in Australia?

Norton:

We went to public schools up until the end of fifth form. I would have been nine or ten. And then my parents managed to send me to a very good private school, Sydney Grammar School, which is modeled on an English grammar school. The teachers there were known as masters, and they wore the sort of gowns that people would wear to a graduation ceremony now. It was a very good education. They worked very well. It was an all-denominations school, which we rapidly learned meant all varieties of Christianity.

Zierler:

Right. [laugh] I'm curious if you had burgeoning interests both in the arts and the sciences, and you felt that pull in both directions even during your high school years.

Norton:

I was a science nerd. I really didn't understand the humanities at all. [laugh] I loved science. I did very well at it. I was a disaster in English classes. I had no idea what was going on.

Zierler:

So the game plan was to pursue a career in the sciences?

Norton:

Oh, yes. I was fascinated by the sciences. I loved chemistry. I spent my early years just playing around with various chemicals. I set up kind of a rudimentary laboratory in the garage, and as a major favor, my parents took me to the biggest bookstore in the middle of Sydney, Dymocks, and they gave me free rein. I could buy any one book that I wanted. I spent the entire morning scouring all of the books, and I finally, after much agony, settled upon J.R. Partington’s Inorganic Chemistry.

Zierler:

[laugh]

Norton:

It’s a big, fat inorganic chemistry book, which I think in retrospect for most people would have been the driest and dullest reading. For me, it was absolutely enthralling. I would read it, and as I'm reading about the chemistry of copper, the chemistry of iron, the chemistry of this, the chemistry of that, it’s all being translated immediately into things that I would do in the garage. It was a real manual of activities. I just carried that book around with me wherever I went. I would always be reading it and scouring it.

Zierler:

So when it was time to start thinking about what schools to go to, were you thinking about chemistry departments in particular?

Norton:

I was thinking about chemistry, but I was a little concerned that I’d be locked away in a laboratory somewhere, and I wanted something a little more versatile. I had no really good basis for choosing anything. I settled on chemical engineering for reasons that I don’t really know anymore. I think it was just a concern that I wasn’t going to be happy in the lab. My parents, at the end of high school, set up a summer break internship at a laboratory. Colonial Sugar Refinery, CSR, was a major industrial concern in Australia. And a friend of theirs amongst the émigré community was an old German chemist, of the old school, and he had a little lab at CSR, and he would do laboratory work for them. I became his lab assistant for a few months. It was fun to be in a real lab, but I think I ended up with the impression that it was too isolated and too separated out from real life. That bothered me. We’re now going back to 1970, so I don’t fully recall. I eventually ended up with the idea of chemical engineering, and I don’t really know why. It was an excellent choice, by the way.

Zierler:

I'm curious, in Australia or at least in your surroundings, if the social upheavals in the late 1960s and early 1970s had reached your orbit.

Norton:

Yes. The Vietnam War was a big deal for us. Australia had conscription—a draft—which sent troops into Vietnam along with the U.S. There were big demonstrations. There was the so-called “moratorium.” I had no part of it. I was too young, and I didn't really have opinions on the matter. I was watching from the sidelines. In a way, it was more traumatic for us than it would be over here in the U.S. There were demonstrations in the street, people marching and protesting, and this was something that was completely new to us. The only time we ever saw a parade was on Anzac Day, which is a national day of celebration.

I don’t know if you know the Anzac story—the Australian and New Zealand Army Corps. This is, we are told in the national mythmaking when Australia became a nation in its own right. It did it by sending troops to Churchill’s misbegotten idea of opening a second front in World War I in the Dardanelles in Turkey, now known as the Gallipoli campaign. The whole thing was a complete disaster. The landings happened at the wrong place. Instead of landing somewhere where there was a flat area where they could advance, they landed beneath cliffs. But they were unable to decide, “Oops, we made a mistake. We’d better get out of here,” and they stuck it out for months, with the Turkish troops at the top of the cliffs, just shooting down at them. We celebrate this. Why we celebrate this, I've never really understood. This is a celebration of a debacle and a defeat. Anzac cookies are a standard recipe. I guess they're Anzac biscuits. “Cookies” is the American term.

Zierler:

I guess after a certain point, the myth becomes sort of more relevant than the reality.

Norton:

Yes. It was only when I came to the U.S. and I realized that in the U.S. people have parades in the street all the time for no apparent reason [laugh] that I could see. And so I began to realize that the idea of a moratorium, a demonstration of people marching in the street, was a routine thing here. Whereas for us, it was really very, very disturbing to see.

Zierler:

With a degree in chemical engineering and with your later work at Shell, entering industry straight after undergraduate was a distinct possibility for you, and making a career in that way?

Norton:

Yes. I did it. I did it with a very particular plan in mind. In the course of going through chemical engineering at the University of New South Wales, we had to take a number of humanities courses. I took several courses in philosophy and a course in history of cosmology, and I found them to be just enthralling. I enjoyed them immensely. I realized that this is probably the direction that I want to take. However, imagine you are a guy sitting in Australia with a chemical engineering degree, what sort of a career do you expect to have? I thought I’d better make sure I have a meal ticket. So I went on the job market. I was the top student in the year. That was not because I have any special engineering powers. It’s because I am theoretically rather good, whereas my colleagues in engineering were very smart, but practical people. I looked terrific in comparison to them, because I got straight A-pluses all the way through. I got more equivalents of an A+ than any other student. I think I got twice as many as the whole year combined. I won medals every year for academic excellence.

I didn't realize at the time that this was rather meaningless for engineering because the real skill of engineering is the practical human side. Chemical engineering in an oil refinery is little different from fixing a broken car in your garage over the weekend. It’s just on a bigger scale. But whatever it is that gets the car back on the road on Monday morning, whatever smarts you have to do that, is what you really need for chemical engineering. I didn't realize that at all. And that’s what I learned at Shell, very quickly. I decided I’d work for Shell for long enough to establish myself in the engineering profession. And then at some point, I would give something like philosophy or history and philosophy of science a go.

Zierler:

So was that essentially what you were doing for Shell? Fixing broken cars on a much larger scale?

Norton:

Yes, that’s exactly it. They put me into “movements” as the first position. An oil refinery has a number of different plants, and they connect together with piping and drains and all sorts of things. The movements position is all to do with those drains. One of the early jobs that I worked on concerned a plant that would treat contaminated water, mostly sulfur contaminated water, “sour water.” The plant it wasn’t working very well, and on one notable occasion it dumped sour water, sulfur-laden water, into the drains. That contaminated water would then go into the Parramatta River, which was a bad thing. My job was to go and fix it. I did fix it, but not just the one particular problem. I fixed it for all time by analyzing the chemistry of the process and realizing that the critical thing was the particular qualities of the water going in: its pH and the buffering powers of its particular composition. I wrote a wonderful report that detailed all of this. I devised new laboratory tests and a complete theory of the process. Then, in the appendix, I mentioned that the particular incident that had triggered the need for my intervention was that someone had dumped a load of caustic soda down the drain. That blocked the stripping powers of the plant. Caustic soda should not be dumped down a drain again. In the end, that one fact was the only useful thing in the whole report. A more practically minded engineer would have figured all this out in one day and moved on to other problems. In retrospect, this made clear that I was a theoretician in a non-theoretician’s position.

Zierler:

So if I could read between the lines, it sounds like what happened was you established yourself in Shell, you did good work there, but then you realized your real love was that history of cosmology class that you took, and you felt like you can go pursue that as a graduate student, and always fall back on something like Shell if that didn't work out. Is that a fair guess?

Norton:

That’s exactly right.

Zierler:

What programs were you looking at? Did you think to move beyond Australia or beyond NSW?

Norton:

Oh, yes. Well, there’s an intermediate step. Growing up in Australia in the 1960s and ‘70s, you're acutely aware that you're very far away from everything. Airline travel now is very cheap. Airline travel then was possible but very expensive. It had only just started to be more common. Up until then, you would go long distances by taking a boat, and it would take a long time. A traditional thing for young Australians to do then was to pack their bag—I packed a backpack—with whatever stuff you have, and buy a one-way ticket to somewhere in Europe and then see what happens. [laugh] So that’s what I did. I got a one-way ticket. This was December of 1976. I got a one-way ticket to London. And I arrived.

Zierler:

This was a solo adventure?

Norton:

Solo adventure, yes. My girlfriend wasn’t terribly pleased about this. We were living together at the time. But this had been a plan for the longest time, and I was determined to do it.

Zierler:

Where did you go?

Norton:

I started off in London, and I stayed at cheap bed and breakfasts and youth hostels. You can live very cheaply if you want to. I had saved all the money I had from working at Shell. I wasn’t spending on anything, so I could fund myself. And I just traveled around and did stuff, and then traveled all the way through Europe. In the meantime, my girlfriend (now my wife of over 40 years) was studying architecture. She decided that she would join in on the fun. You have to take a practical year in an Australian architecture degree, so she came to Vienna, where her aunt lived, and she moved in with her aunt, and then found a job with a Viennese architect. She speaks enough German, enough Austrian German to get by, so she could work there. And having English of course is an enormous advantage in an international architect’s office. We reconnected and started traveling together.

I went all over Europe and I went to Israel and visited my relatives there. I sent a communication to my wife—this was a difficult time, because there was no email. We would communicate by complicated means. Anyway, I send her a communication—I don’t know how—saying, “Israel is wonderful. You have to come.” And so she came over. We were staying with my aunt who was an Auschwitz survivor. When there are Holocaust deniers, I think I have as close to first-hand evidence as you can get that it was real. She personally survived and had the tattoo on her arm. It was pretty horrific. She suffered medically for it, as well. She had some sort of spinal deformation that resulted from an illness in the camp. Anyway, that’s another story.

Zierler:

Did you go to places like Einstein’s birthplace and the Niels Bohr Institute? Did you indulge yourself in that way during your travels across Europe?

Norton:

I wasn’t an Einstein scholar at that point. I hadn’t really taken up the interest. However, I had ended up in Hamburg because I had met some people from Hamburg in Germany, and they invited me over. And so, OK, off I go. You know, the freedom with which I traveled now in retrospect is just extraordinary, at a time when traveling was not as easy as it is now. So I'm in Hamburg. And then where do I go next? Well, I'm in the far north of Germany; I should go south. I knew that Einstein was born in Ulm, so I figured I’d go to Ulm. The people I was staying with went to the university, and there was a system where students who were driving various places would give you a ride, and you would help cover the costs. So we drove all the way across Germany through the night and then they dropped me in the middle of Ulm. I started walking around, saying, “Well, now what?” I heard there was an Einsteinstrasse, so I went to Einsteinstrasse. It turned out to be just a very new, very modern street—it was more something in name only. There was no real Einstein stuff there.

What was more meaningful for me was when I went to Berlin, to try and find where my mother had lived. I wasn’t very well organized in doing it. I stayed at the youth hostel in Berlin. This is 1977, so the Berlin Wall is still up. And so I made the trip across the wall into East Berlin just to walk around and have a look. I somehow got the idea that my mother had lived in East Berlin, but that turned out to be a mistake. Ost-Berlin to [laugh] put it better in German. When I worked for Shell, they sent me to Singapore, and I bought a Rollei 35 S camera, a tiny little portable 35-millimeter camera, and I would take one or two carefully selected slides on Kodachrome a day during the trip, because Kodachrome was expensive; you couldn't just pop off a roll on a whim. And so as we crossed over the wall, I took a photograph looking all the way down the wall. It was extremely foolish in retrospect because I was making the trip from West to East Berlin. [laugh] And so if I’m going to do it, I should have done it going in the other direction.

Zierler:

[laugh]

Norton:

But I wasn’t really thinking about such dangers. Fortunately, nothing bad happened. I subsequently have been back to Berlin quite a few times, through my work with the Max Planck Institute. And in subsequent visits, I got the address from my mother as to where she lived, so I could find. It was actually a very well-to-do street directly off the Kurfürstendamm in West Berlin.

Zierler:

Was she ever able to recover assets from her family that were lost to the Nazis?

Norton:

I think so, because she worked in Germany for a while. There was some kind of social security plan, and I think she could recover what was owed in social security payments. But I think that’s all she was able to recover. She had a lawyer representing her. I don’t know the details

Zierler:

Were you considering graduate school in Europe, or you knew you were always going to go back to Australia to pursue your degree?

Norton:

I was drifting along. There was no hurry in life. I'm a young man. [laugh] It’s very exciting. I'm walking the streets of London, and for the first time, I'm standing in the streets that were named in the Monopoly games that we always used to play. Monopoly is an American game, and in the U.S., the streets are named after those in Atlantic City. We had the English version where the streets are named after the streets of London.

Zierler:

You know, for some reason, my family had the British Monopoly game too, so I know those streets better than I know the streets from the American game. I don’t know why we had that one, but we did!

Norton:

So you know the streets very well. It’s amazing to actually walk on those streets. After a few months, I realized that I needed to do something to plan for the future. I had gone out to both Cambridge and Oxford. You just hop on the train, go out there, walk around and find a youth hostel. Youth hostels are cheap. You get company there. There are other people in youth hostels. So it was quite pleasant. So I determined to go back to Cambridge. I liked the look of the place better than Oxford. It was a completely arbitrary choice. I took out non-member leave to attend lectures in the Easter term. And that just gave me an authorization to go in and sit in on any lecture that I wanted. It gave me library privileges as well. So I got the catalog of lectures in philosophy and HPS, and I attended a large array of lectures. When I was in Vienna, ahead of this, I was getting bored, hanging out in Vienna with my soon-to-be wife, trying to think of what to do, so I found an English language bookstore. And they had books on philosophy. There was a book on Wittgenstein which I bought, and they had Ayer’s Language, Truth, and Logic. I don’t know if that means anything to you. It’s one of the founding documents of logical positivism. It’s a polemic against metaphysics. I read both of those books, and they were profoundly affecting. I read Ayer’s Language, Truth, and Logic and I personally experienced the “elimination of metaphysics,” (as Ayer triumphantly proclaimed it) which in retrospect is rather comical, because I had no idea what metaphysics was.

Zierler:

[laugh]

Norton:

Anyway, with that background, I went to Cambridge, and I sat in lectures. So I remember—

Zierler:

And mostly history of science and philosophy of science kind of lectures?

Norton:

Yes, some, but mostly straight philosophy lectures. I sat in on Elizabeth Anscombe’s lecture. You don’t work in philosophy, but she’s a very significant figure. I sat in on her lectures on metaphysics, which were, in a way, rather disappointing. One of the major points that she had to make was that she knew Wittgenstein. And, OK, fine, you knew Wittgenstein. You told us once. We know. But it always seemed to keep coming back. Or maybe I remember it more vividly because I’d read about Wittgenstein and this was a point of contact. And then she would talk at length about metaphysics, and I was sitting at the back just thinking to myself, “Well, don’t you know, it has been eliminated?” [laugh]

Zierler:

[laugh]

Norton:

I attended a whole range of lectures. Some of them were interesting. Some of them were in logic. There was a class with Dr. Smiley on definite descriptions, which I thought was rather neat. If in logic you say the cow is brown, are you asserting the existence of a cow, or are you making a generic statement such as you would make when you say that unicorns have a single horn? You're not asserting the existence of unicorns, but merely what their properties would be were they to exist. There was a huge debate between Russell and Frege over this, and it went for the whole term. There were lectures by Harrison on utilitarianism which I found rather fun. His lectures were dramatic. There was a lecture by someone fairly junior on Kant, which I found unintelligible. That’s something about Kant that’s still true today, even though I should know better.

Oh, then there was a lecture on other minds in a lecture series. There are various forms of skepticism in philosophy, and one of them is that you doubt the existence our world. Another form of skepticism is that I doubt that you have a mind. Even though all the exterior manifestations is of a thinking person just like me, how do I know that you have a mind like mine? Now, all of this was rather discouraging, because I would go into these lectures knowing something—“you have a mind”—and I would come out knowing less. And this did not seem to me to be progress. [laugh] I also attended lectures in history and philosophy of science. There was a sequence of lectures given by different people on different topics. I remember there was a Freud lecture, for example. And that was wonderful, because I came out knowing more. And so that decided the matter that I was going to do history and philosophy of science, not straight philosophy.

Zierler:

And you wanted to go back to Australia to do this?

Norton:

Yes, that was the plan. I didn't really have any other options. We were well established in Australia. We had lived in a house that had been owned by my wife’s parents, and could go back to it. We had roots there. And the plan was always to go back to Australia. I knew the system there. It’s easy to do postgraduate work the first time around. There were minimal fees. If I wanted to do a second postgraduate degree, that wouldn't be the case. It was easy for me to get a Commonwealth Scholarship and go back to the university I knew. In Australia at the time, there was much less of a concern that you go to the right university. There’s a small number of universities in a government-run system, and the major university in your town is a perfectly fine place to go. So I found it unusual when I came to the U.S. that people would always append where their doctorate was from. “Don’t worry, sir. I'm a doctor… from Harvard.” Right?

Zierler:

[laugh]

Norton:

I thought, “Well, why say that? You're a doctor. That’s good enough.”

Zierler:

Right, right. [laugh]

Norton:

It’s a very different system.

Zierler:

Now, did you immediately gravitate towards history of physics at first, or did you take a more generalized approach?

Norton:

I was interested in philosophy—philosophy of physics—and I determined fairly early on that I wanted to do something in that area. I was interested in time, because time seems mysterious and hard to understand. That was the topic that I worked on. I was assigned a supervisor. He got me starting to read papers on time. It became very clear, early on, that Einstein was the central figure. There was a huge amount of work that he had done, of great importance, to do with time.

Zierler:

So your program was a philosophy program. It wasn’t a history and philosophy?

Norton:

There was a school of history and philosophy of science, specifically. I take it you know there are departments of history and philosophy of science. There are only a small number of them, and they're dwindling for various institutional reasons. My old school no longer exists. It was broken up a few years ago. But when I was there, it was a good department. They immediately put me into what would be the equivalent of graduate seminars in order to bring me up to speed. I was increasingly dissatisfied with the philosophy of science that I was learning. I think the best way to convey my dissatisfaction was my advisor was really doing his best to help me along, and he was very sure that the rhetoric of the matter was the key thing. So at one point he gave me the sage advice that you give your students, He said, “It’s not what you say; it’s how you say it.” And I thought that was appalling. And I had a feeling that too much of the philosophy of science that I was doing had that character.

We were reading philosophy of science in the Popper/Kuhn/Lakatos/Feyerabend tradition. And it was fun. I remember reading Kuhn’s Structure of Scientific Revolutions and finding it to be just an enthralling read. But I was not comfortable with the idea that it was correct. That there was a relativism about scientific knowledge. It didn’t square with security of the science I knew. Social constructivism was doing very well at the time. I was sure it was wrong and reacted quite strongly against it. The constructivists were saying that the actual content of science was, in effect, just made up like a good piece of fiction, but that it achieved its status just because enough scientists decided to agree on it. That was too far from the science I knew.

Zierler:

Would most faculty at that point have defined themselves as postmodernists?

Norton:

No, there was quite a mix. The one faculty member who stood out to me as the most valuable was David Oldroyd. He worked in various areas including history of biology. And he had a two-term course on the history of the methodology of philosophy of science. He called it “isms.” And that for me was really profoundly helpful. He started with antiquity—Plato, Aristotle—and went all the way through to 20th century figures. He asked, just what were their views and how did it all fit together. His attitude was one of curiosity and openness. He became a model for me of how to work. Then there were others. There was a Marxist, but I found the whole Marxist approach unappealing. I didn't find the ideas convincing. There was a kind of guardedness about them that I found difficult. It was very hard to get them to say clearly what they meant. They spoke in long, elaborate flourishes that were just confusing.

Zierler:

How did you go about putting your dissertation topic together?

Norton:

Well, it was actually given to me. As the dissertation work progressed, I moved away from time, and got closer and closer to Einstein. My dissertation supervisor, John Saunders, had written a historical dissertation on special relativity. So he said to me, “Well, you should do general relativity. It hasn’t been done yet.” And so that was basically it. I didn't know enough to know any differently. I taught myself a little bit of general relativity when I was in Cambridge. I’d met an American in the philosophy library at Cambridge. He told me that relativity theory wasn’t that hard. So I went to the bookstore in Cambridge and picked up a general relativity textbook. I had been working through it. I found it very hard—in retrospect, that was because it was a terrible textbook, and it was very hard to learn from. The other thing I had done at Cambridge, by the way, in the evenings, was I had become convinced by Ayer and Wittgenstein and others that logic was important to the proper practice of philosophy. So I was just reading in formal logic and metalogic, which turned out to be a very useful thing to read.

Zierler:

I'm curious if you were able to lean on your undergraduate education in chemistry as something that was helpful for your philosophy of science work.

Norton:

Well, the mathematics was very helpful. Chemical engineers have to learn a great deal of mathematics, so I was quite up on all sorts of mathematical techniques that engineers know. We had learned the theory of integral transform, such as the Laplace transform, for example. That was helpful. Later on—this comes much later on—I started working on thermodynamics and statistical mechanics. There, my engineering background was crucial, because I had a solid foundation in thermodynamics, and that’s hard to get. Thermodynamics is a very strange science, and until you kind of get it, it’s completely mysterious how the thing fits together. But when you get it, it suddenly becomes very clear and very beautiful. My acquaintance with Laplace transforms made it easy for me to grasp a hugely important moment in quantum theory in the early 1910s. Ehrenfest and Poincaré then demonstrated that quantization was unavoidable if we are to treat heat radiation adequately. The quantum was coming whether we wanted it or not. The core of their argument is just the inversion of a Laplace transform. They didn’t present it that way since the independent theory of Laplace transforms did not then exist. They had to invent it as they needed it. Profiting from the later work on transforms, I saw it even before I realized that this was their argument.

Zierler:

To the extent that you're able to separate these things, how much was your dissertation about Einstein and how much was it about general relativity?

Norton:

It was about Einstein’s discovery of general relativity. It was just a straight plodding narration. In contrast, special relativity essentially dropped out of the sky. Einstein published his paper in 1905. We have very little earlier material on where special relativity came from. I've worked subsequently pretty hard on trying to tease out what we have. Others have as well. But there are no earlier papers. There’s correspondence, and the correspondence has fleeting clues. There are lots of later remarks and correspondence as to how Einstein found special relativity. And if you put all that together with a healthy dosing of speculation, you can come up with a pretty good story, which I've done in a long paper. It’s a great paper. I recommend it to you. [laugh]

Zierler:

[laugh] And so once you settle on this topic, are you doing a crash course in German, or you're already prepared in that regard?

Norton:

I knew enough German at that point to get by. I had had—let me think—five years of German at school, and so that gave me the basis of the grammar. And then I had lived in Austria and traveled in Germany a fair amount. So my German was vaguely passable. You know how I chose German over French? When I was in high school, we had to make a choice. I had classes in Latin, German, and French—all three languages—and at one point, I had to choose between French and German. I went to that Partington book, the chemistry book that I loved so much, and I opened at the front, where there was a bibliography. I counted the number of French references and the number of German references.

Zierler:

No contest.

Norton:

There were more German references than French references, so I thought, “OK, I'll continue with German.” It was a good choice in the end. Let me put the pieces together. You'll see how it worked. In the case of general relativity, Einstein’s work on the theory spanned a period of seven or eight years in which he published repeatedly. The first published paper is in 1907. It has his earliest fleeting ideas. Then there are more in 1911, 1912, 1913, 1914, 1915. It was a steady stream of papers. This is the days of interlibrary loan. So I went to the interlibrary loan librarian, and filled out all the forms—“I need this paper. I need this paper. I need this paper.” They were papers in fairly obscure old German journals, so they weren’t easy to get a hold of. And they eventually came in. One of them Einstein published in 1912 in a journal called the Vierteljahrsschrift für gerichtliche Medizin und öffentliches Sanitätswesen, the quarterly for—what was it?—for sanitation and public health. This was the paper in which the Mach principle appears for the first time. I'm sure you know the Mach principle …

Zierler:

And this is because more respectable journals simply would not accept anything from Einstein?

Norton:

I didn't know the story for a long time, and I'll tell you in a moment why it was there. So I handed this over to the interlibrary loan librarian, and she looked up and me and she said, “You know, some people will do anything to get published.” [laugh]

Zierler:

[laugh]

Norton:

I thought that was lovely.

Zierler:

[laugh]

Norton:

The story turned out to be that Einstein had a good friend I think in Zurich by the name of Heinrich Zangger, and Zangger worked in that public health area. This was a festschrift in his honor. And they had no doubt asked Einstein, “Would you contribute to this?” And Einstein just gives them what he’s working on. He’s got a repository of thoughts, and he just gives them to whoever asks, and that’s it. You want something else? You won’t get it. And so I think they just took it. In 1912, Einstein was not yet the celebrated public figure where anything would be terrific, but he was established. He was a Herr Professor. He had that kind of status already which would make him at least somewhat revered. He hadn’t yet got the call to Berlin in 1914 where he becomes a director of a Kaiser Wilhelm Institute, where he’s then someone major. Anyway, that was the story. I had those papers, a big stack of them, and I just started reading them, one after another. Every time I’d come across something I didn't understand, I’d go away, find physics textbooks, and read up in them.

I had put in a fairly concerted effort to come up to speed in general relativity. My advisor had told me about “Misner, Thorne, and Wheeler.” I don’t know if you know any general relativity; you know Misner, Thorne, and Wheeler? The “big black book,” where a generation learned their general relativity? OK, so I got a copy of Misner, Thorne, and Wheeler, and I just sat down and started plowing my way through it. You've probably got a sense here of how isolated it is to work at that time in Australia. I'm working by myself. I don’t have anyone who I can really talk to. At one point, I was puzzled by indices in an expression enclosed in square and round brackets. I went to my supervisor to ask what the notation meant. He didn’t know. I soon learned that these were merely anitsymmetriziation and symmetrization operators, so widely known that the text didn’t see the need to explain them. It was intellectually lonely, but it did teach me one important skill: intellectual independence and how to solve problems myself.

Zierler:

Is there a physics department that you felt like you can go and ask questions if you wanted to, or that was not even available to you?

Norton:

There was a physics department with some fairly good physicists. I eventually went and took a course with a friendly physicist John Shepansky in general relativity. I learned rather little that I didn't already know. He was a really useful resource only once. I had gotten into a major 1913 paper that Einstein had written, and there was a section I simply couldn't follow. I was just confused about what was going on, and I took it to him. I can still remember the meeting. For me, the photocopies of the papers were precious documents, because they were so hard to get. And I took the photocopy to him while he was eating lunch. He was looking at the precious paper and eating his lunch and the food was falling on the paper, and I wanted to brush it off, and I [laugh]—I was very unhappy with this.

But he said, “Oh yes, I know what this is” and he rattled off what it was. What Einstein was doing was working with stress tensors and stress-energy tensors, and I hadn’t fully absorbed the profound importance of these structures in general relativity. Because it doesn't really come out much of the textbooks, unless you know how important they are. That was enormously helpful. I then went away and really came up to speed on them. Recognizing their centrality has carried me through a lot of research since. In fact, I wrote a paper just a couple years ago on the role that these structures played in Einstein’s early work on general relativity. Because I realized that their importance hadn’t come out very clearly in much of the secondary literature. I thought there was a real need just to lay it out.

Zierler:

I'm curious if you dealt with the concept of genius, generally, as a way of trying to understand where Einstein was coming from.

Norton:

No. I didn't—I found the idea problematic. My view of it was that Einstein was engaged in an extraordinary enterprise, but one that is intelligible to everybody else. You can understand. I wanted to know how some guy sitting by himself in whatever place he was--in Bern in 1907, in Zurich, in Berlin—can come up with an extraordinary idea that gravity is the curvature of space-time. To simply say, “Well, he was a genius and so he saw it,” I thought was unhelpful, uninteresting, and not what happened. What happened was that Einstein had a very well-developed research agenda, and he was carrying it through step by step by step. I've spent a lot of time reconstructing the research agenda that he had.

You've heard of the principle of equivalence in general relativity? The principle of equivalence nowadays says you can always transform away gravity. And this is helpful in understanding the structure of space-time. This isn’t what it did for Einstein. I noticed this very early on. You know what it’s like when you're writing historically. You make a claim. Now I'm disciplined. I immediately check the source. I don’t leave checking the source until the paper is finished. I check the source at the time and put in a quote. Right? And so I'm writing perfectly ordinary material in my dissertation. I come to the principle of equivalence. I know what it says. I flip to a standard Einstein source for the quote. It doesn't say that. Well, I picked the wrong source. And I checked every source I could find. None of them said what I thought the principle said, that you can transform away gravity by acceleration. It was completely mysterious. What Einstein’s version said was something different. It said that if you uniformly accelerate yourself, you recover a homogenous gravitational field. So imagine you're off in space somewhere; you uniformly accelerate and free bodies around are left behind. They accelerate away from you.

What appears around you is a homogenous gravitational field that drives them. So why is this useful, then? This is useful to him because in 1907, he had been trying to relativize gravity. He had the special theory of relativity. Johannes Stark then commissioned him to write a review article of the principle of relativity, as it was then called, covering its application in all the branches of physics. Einstein goes through and explains how electrodynamics fits with the relativity principle; how mechanics has to be adjusted; E equals MC squared; mass depends on velocity; and how thermodynamics can be made to fit with the principle of relativity. As a result, the first relativistic thermodynamics appears. And then comes gravity, and Einstein can’t get it to work. He convinces himself it doesn't work. So he has got a new research project, which is to figure out what a relativistic theory of gravity would look like. The difficulty was that in the relativistic theories he had cooked up, all bodies weren’t falling with the same acceleration. Give a body a sideways motion, and it would fall with a different acceleration than one that didn't have the sideways motion. That was a violation of Galileo’s result that all bodies fall alike, and he felt this was just untenable.

It is possible to understand why he thought this. There’s a story that I can go into about why that was in fact plausible that he would do that. That means that he had a problem. He didn't have a theory of gravity for the review. This is where the principle of equivalence comes in. The beauty of the homogenous gravitational field that is produced by uniform acceleration is that it satisfies the Galileo principle. All bodies fall with the same acceleration no matter their sideways velocity. So he tried to recreate that in special relativity. It was an easy thing to do. He goes to what we would now call Rindler coordinates, although at the time they would have been better described as Born’s. You know, Born actually did the earliest work on this while he was in Göttingen. So he now had one example of a relativistically clean gravitational field. That’s what the principle gave to him.

The project for the years that followed, up until the end of the summer of 1912, was simply to generalize these properties. So he has a definite research agenda. You write down what this field’s properties are in the particular case. And then you ask, what is the generalization that will give a more general theory? One of the things you discover is that the speed of light varies linearly with distance in the field. So c, the speed of light, is a linear function of distance. Now that linear dependence is a special solution of the Laplace equation. So it’s a natural thing to assume that you recover more general fields by replacing that linearity with any field that satisfies the Laplace equation. He has now concluded that the speed of light is the gravitational potential and that it satisfies the Laplace equation. That was the way that he was proceeding. So flashes of insight, moments of brilliance? Absolutely. But all happening within a well-articulated research agenda.

Zierler:

So this is not to deny Einstein’s genius. What you're saying is that that’s not a particularly illuminating way of understanding his process.

Norton:

Precisely. You can identify that there are moments when suddenly he makes a step that you think, “Wow. Oh! That was extraordinary.” One of the things I learned very early on is that you don’t try and predict what he’s going to do next, because he almost never does what I think he would do next. He has damn good reasons for why he does the next step, even though they're not always obvious.

Zierler:

John, on that point, I wonder if you could explain both scientifically and perhaps as an insight into Einstein’s process, why the need to generalize these properties. Why is it not enough or sufficient to just understand what the properties are? Why is there a need to put them in a larger systematic framework?

Norton:

Well, the principle of equivalence gives him one case only. A homogenous gravitational field is one that has the same acceleration in the same direction at all points in the field. That’s only a special case of the gravitational field. You might be interested in the gravitational field of the Earth, for example. It points in different directions in space and weakens as we move away from the Earth. So what is the equation that governs that? What is the physics that governs that? Well, he has discovered that the speed of light functions as a gravitational potential. It has the same role in the special case as the gravitational potential does. So let’s assume the same thing is true for the gravitational field of the Earth, where now the gravitational field of the Earth points in all directions and [laugh] intensifies the closer you get to the Earth. So that linear dependence is not going to work. The linear dependence is going to give you a constant gravitational field. It’s the derivative that gives you the intensity of the field. So he has to find a way of generalizing it. And that was the program.

Zierler:

What’s the chronology that your dissertation covered? What years does it cover?

Norton:

1907 to 1915.

Zierler:

And then obviously 1915 is a pretty sensible place to stop, because other big things are happening then.

Norton:

Yes, but 1915 is when Einstein settles on what we now know as the Einstein gravitational field equations. It’s a natural stopping point. He doesn't have lambda yet. He doesn't have the cosmology constant until 1917. So things keep developing, but that’s a natural stopping point.

Zierler:

In terms of your contributions, I'm curious—how much does your dissertation reveal about Einstein as a person? Does it delve into that very much, or is it mostly technical?

Norton:

I've stayed away from the Einstein personality, the private life. It’s not something where I have expertise as a historian. I've not really done that kind of personal history. I'm well-equipped to do it. There has been a huge amount of work that has been done since on his private life. You probably know that Einstein’s private life was rather colorful.

Zierler:

Oh, yeah.

Norton:

He had girlfriends and mistresses. This stuff was all kept secret. The documents that demonstrated it were embargoed by the Einstein estate. That’s Helen Dukas, Einstein’s secretary; Otto Nathan, his lawyer; and Margot Einstein, his—I guess it’s a step-daughter, the daughter of his second wife, his cousin Elsa Einstein. When Helen Dukas died in early 1982, the Einstein estate was dissolved, and Einstein’s papers were collected and shipped off to the Hebrew University, in accordance with Einstein’s will. They made the decision that they would open the papers, come what may. It was a good choice.

And so of course all of these stories came out. You start reading the letters, you start reading the papers, and you realize—you discover that Einstein and his first wife, Mileva, had an illegitimate child prior to their marriage. This of course was something of a disaster for his wife’s private life. Various other salacious stories that I guess you know about appeared. My overall reaction to it is that we're applying odd standards to Einstein. This is something I've written on Einstein intellectually. He was very much a 19th century thinker. The stuff that he produced, in large measure, was harvesting the scientific achievements of the 19th century. Special relativity is a direct product of the electrodynamics of the 19th century. One of the major mathematical advances of the 19th century was non-Euclidian geometries and non-standard geometries. Einstein applies that to gravity. That’s eventually what happens with gravity.

There’s so much that he’s doing that is fulfilling the 19th century ambitions. Einstein is seeking a unified field theory. This has been a major theme all the way through the 19th century. The unification of forces was a major goal, certainly going back to Faraday and perhaps even earlier. Einstein’s ideas about causation were 19th century ideas. Causation lives in determinism, it was then decided, in the determinism of the world. Quantum mechanics is not deterministic. This bothers Einstein immensely. Now, that’s just the beginning of what bothers him. But he’s very clear all the way through that the indeterminism troubles him. He’s a 19th century guy, intellectually. It’s very easy for me to think that he’s also a 19th century guy in matters of his private life. And there, the rules were rather different. Marriage was at least in some circles a public event, a contract between people to produce children. And as long as that contract is fulfilled, what you do in your spare time is your own business. That was clearly in some strata a way you would think about things. And that was Einstein’s life. So I think nowadays we're horrified by this. We don’t want to live that way. [laugh] Right? So we think of it as some kind of profound moral failing. I think it’s just a different way of living your life.

Zierler:

To what extent have you looked into, in your research, Einstein’s spiritual life? In other words, famous mantras like “God doesn't play dice with the universe” or perhaps looking for a spiritual or even religious underpinning to his drive to find a unified theory and what that might mean. Have you looked into that in any systematic way?

Norton:

I haven't been terribly interested in that. If you want to read about that, Max Jammer is the source. He wrote a very elaborate treatise on this. Here perhaps I should say something about Einstein as a philosopher. Remember, I'm a philosopher. It’s one of the things I do. I do a lot of work in just ordinary philosophy of science. Einstein was not a great philosopher, but he didn't aspire to be. I know there’s one letter someone wrote to him in which they sent him some question in philosophy. Einstein replied saying, “You know, I'm a physicist, not a philosopher.”

Zierler:

[laugh]

Norton:

Now, that said, Einstein read philosophy and got a lot out of it, and understood it very well. But he was not a systematic philosopher. There’s a way to understand the difference. Think about a chef in Northern Italy who is determined to capture the true cuisine of Northern Italy. They’re going to have a very systematic cuisine. They are going to follow authentic recipes, and they will keep to the canon, as it were. Now, imagine me going home at the end of a long day, tired, and opening the fridge door, and saying, “What am I going to cook tonight?” [laugh] Now, I might poke about in a cookbook and say, “Oh, here’s this Italian recipe. It’s got pasta and garlic and some olive oil. Hey, I'll do that! I think that’ll get me a good dinner.” That's the way Einstein, and in fact many physicists, use philosophy. It’s a repository of interesting ideas that might be useful and fertile. They can be used independently of whatever larger system of philosophy contains them.

Zierler:

But not scientifically useful, you're saying.

Norton:

Yes, scientifically useful. That’s exactly it. So the Mach principle comes directly out of Mach’s rather extreme positivist analysis. Nowadays, we look at Machian positivism and say, “Well, this is so extreme as to be unworkable. This is an unworkable philosophy. It in no way applies to modern science.” But you can pull out of Mach an idea that was useful to Einstein. If inertial forces have to have an observable cause, then you end up with the Machian-style physics that Einstein was initially looking for. I did some fairly close work on this, and I became convinced that what Einstein thought Mach said, Mach didn't say. Mach was enormously evasive about what he actually said. Curiously, Mach’s ideas on inertia were regarded by other people at the time as crazy ideas. They were inadmissible, a priori science. And it was really only Einstein that made them respectable. In the early literature of the 1910s, I came across now prominent philosophical figures like Philipp Frank and Moritz Schlick condemning the Mach’s principle. Schlick said that the idea was so obvious that he himself had had the idea when he was a primaner, which people tell me is something like a sixth form student. Loosely, that means a high school student.

Zierler:

John, I'll ask a question that might bring you back to your thesis defense. What do you think your principle contribution was with your dissertation?

Norton:

My principle contribution was simply to learn fairly well the chronology of what Einstein did. There’s a lot missing in my dissertation. It’s not a terribly useful work for anyone who wants to use it. Now, it simply lives at the university and I've never gone back to it.

Zierler:

But in terms of your source base, it’s foundational, because you were really looking at papers that had not been worked in before.

Norton:

Yes. Well, that’s exactly right. And so that’s what I learned from the research needed to write it.

Zierler:

Which is remarkable, by the way. It’s like, looking back, how could that be?

Norton:

Well, I got completely immersed in Einstein’s thinking and what he was doing. The positive contribution that I made towards the end was to grapple with the following problem: Einstein insisted that his theory was a generalized theory of relativity. That is, it embodied a generalization of the principle of relativity. It seems fairly clear now that it doesn't do that. And so it’s mysterious—how could he have this heuristic that overtly was doing all this tremendous work for him, but it actually turns out to be contradicted by the final theory? What I was arguing was that that wasn’t the only heuristic. He had a large number of additional heuristics, some of them fairly prosaic physical ideas. The principle of equivalence is telling him that gravitational phenomena essentially live in the spacetime as a part of spacetime.

That’s already a profound idea. He has ideas about what the governing equations have to look like. They have to reduce down to Newtonian theory in appropriate circumstances. He needs to have energy conservation working. There’s a whole bunch of things like that. Energy itself has to be a source of the gravitational field, and that will force non-linearity of the gravitational field equations. So there’s all this stuff going on, and I argued that they are what’s really doing the work for him.

Zierler:

In retrospect, your postdoc looks to be just the perfect place, the place where you inevitably will wind up, working with the Einstein Papers Project in Princeton. Was that the case? Was it as seamless as it may seem?

Norton:

No, not at all. I was quite happy just to hang out in Australia. I had no ambitions to travel the world anymore. I had spent a year traveling around Europe, and I was fed up with traveling. But my wife is more ambitious. She finished her architecture degree and she decided she wanted to do an urban planning degree at Columbia. And so off she goes, and it’s basically “I’m going…”—we were married then. We got married in London in 1977, in that year while I was traveling around. And so off she goes to Columbia, and she says, “Well, what are you going to do? Are you coming too?” [laugh]

Zierler:

[laugh]

Norton:

So I started researching, “Who’s out there?”

Zierler:

Were you thinking you were going to pursue an academic career in Australia? Was that the plan?

Norton:

Yes, I thought I would. Career preparation is not something that was being done in my department. It was just, “You're going to get your degree. You've done really well. You've been one of the best students we've had. What are you going to do now?”

Zierler:

“Good luck.” [laugh]

Norton:

Yes. Coming to the U.S., it was astonishing to me to discover that there’s a whole program of assistance. I've been doing this now for 40 years. We help our students. We prepare them for the job market. It’s amazing.

Zierler:

In terms of your research, was there no reason to come to Princeton prior to this?

Norton:

If I had been a better historian, yes. But it never really occurred to me that I should do that. I set myself the problem of, “Here’s a large collection of papers that Einstein wrote that document his discovery of general relativity. Make sense of them. Figure out what was going on.”

Zierler:

But there are relevant papers in that topic at Princeton.

Norton:

Oh, yes. But it wasn’t the papers that transformed me as a scholar. I’ll get to it. So anyway, so I looked around—“Who can I work with?” I started writing to people in the U.S. to see who I could come and attach myself to while my wife was studying at Columbia. Most people didn't respond to me. Clark Glymour—I don’t know if you know the name—wrote me a nice two-page letter which was very helpful. I'll always remember his kindness for doing that. But John Stachel at Princeton was very forthcoming. He said he’d host me. I could come and hang out there. I take it you know who John Stachel is.

Zierler:

Sure.

Norton:

Or perhaps for the purpose of the interview, I should say who he is.

Zierler:

Please.

Norton:

John Stachel is a professor of physics at Boston University who has strong philosophical and historical interests. He became the first editor of the Einstein Papers, and starting in the late 1970s, he was working on editing the Einstein Papers for publication with Princeton University Press. In our little community of Einstein scholars, John is the experts’ expert. He is the ultimate source that we go to solve our biggest problems. So I show up there in Princeton, at the Press offices. My wife goes to Columbia. I had a small amount of teaching in my old department through the end of 1981 that delayed me. At the end of 1981, I hop on a plane with a suitcase full of all my books, weighing a ton [laugh], and I arrive first at Columbia, where my wife is already established, and then go off to Princeton. There’s an annex at the back of Princeton University Press, and that was where the Einstein Papers were situated. John Stachel was there. He had a complete photocopy of the microfilm version of the Einstein Papers, which is not the complete thing, but it was enough. And he was working on volume one of the Einstein Papers at the time.

Zierler:

And so what exactly was your work? Were you working in some ways as an archivist as well as a historian?

Norton:

No. I was self-supported. I was just a guy who sat at a desk [laugh] in the Papers. But for me, it was transformative, for two reasons. First, I had access to all of the Einstein Papers. John Stachel very generously said, “Here they are. You can read as much as you want.” And there are huge file drawers full of them. It’s a whole wall of file cabinets. It was a major resource. But then more importantly, he talked to me. And he’s an instinctively good historian. I really learned how to do history from him. And also, since he is a good relativist, how relativity theory is done well. That’s how I realized how much I had lost by being in Australia. I had never had an apprenticeship experience. It was just me by myself figuring out what I wanted to do. And that was when I learned that education is apprenticeship, something that has carried me all the way through my later life as a professor.

Zierler:

Can you share a story or an exchange that really illustrates what a good mentor he was in that regard?

Norton:

Yes. I don’t remember the precise details, but I remember the nature of the interaction. I’d have some interesting and exciting idea, and he’d be there, and I’d say, “Hey, John. Look at this. What do you think of this?” And his instant reaction would always be, “Oh, that’s interesting. Why don’t we check the sources?” [laugh] So he really drove home for me that historians don’t know anything intrinsically. You only know what the sources tell you. Now, this might seem a trivial insight, but one of the differences I've noted between historians of science—historians of physics—and physicists working in the history—is that they think they know things historically in virtue of being a physicist.

Zierler:

Right, right.

Norton:

The clearest example of that is not a Stachel example. I guess you're too young to have seen the fuss first hand when Kuhn produced his black-body book in 1977. He suggested that Planck really wasn’t quantizing the radiation field or Hertzian resonators in the way that we now think, it created this incredible furor, because everybody knows that’s what Planck did. But you ask the sources—go back to the texts—all you have to do is read Planck’s original paper, and it’s clear. He didn't think he was doing that. He thought he was doing something else. It’s completely clear. But there was this huge debate, because physicists know things like this. Now there’s a mechanism here, because they know things like this because their teachers told them. And the teachers knew it because their teachers told them. There’s an unbroken chain to the people themselves.

What they don’t take into account is that this chain can get garbled along the way. And also, there’s also a lot of shortcuts taken on the way. I remember being contacted around 20 or 25 years ago by a physicist who was very bothered by one of his colleagues who was saying odd things about the foundations of relativity. And he wanted to know from me—“So, what is the evidential foundation of special relativity?” It turns out that the evidential foundation in almost all the introductory textbooks is just flawed. The Michelson-Morley experiment doesn't do it. As John Stachel pointed out, for Einstein that experiment was only evidence for the principle of relativity, not the light postulate. The Michelson-Morley experiment is entirely compatible with a Newtonian emission theory of light. That theory violates the light postulate. The evidential foundation is a great deal more subtle, but the subtlety of it is too complicated to give in an introductory textbook. This happens over and over and over again. There are numerous examples of this. The textbooks tend to contain the simplified evidential case for the theory that just basically says, “Young student, this is all you need to know. This is the basis of the theory. Here’s some reasons why this is the correct theory. Now, get on with it. You've got to get to chapter ten. That’s where the novel material is happening. That’s where your career is.” So it makes perfect sense for training physicists to do new work. But it makes for bad history. Anyway, Stachel was enormously helpful there. You're wondering what I got out of the archive. Well, a number of things. First, you know how to work archives.

Zierler:

Absolutely.

Norton:

They are completely overwhelming experiences. You go there with a goal of finding a particular thing, because otherwise you don’t know what to do. Of course you never find it. If you are attentive and open, you find something else. And that was the basic story. I went there with the principle of equivalence problem. I mentioned that to you earlier. Einstein’s version of the principle was different from everybody else’s, and I wanted to know why.

Zierler:

Did you look at this as an opportunity to expand the dissertation, or did you look at this as a new and self-contained project?

Norton:

This would be a new paper. I had written the dissertation. It was over. It was overwhelming to encounter the massive size of the archive. What should I do to find something? The documents were numbered—one-double-oh-one, one-double-oh-two, one-double-oh-three. I just went to the folders, looking at one-double-oh-one, one-double-oh-two. I'm just looking for clues. To cut a long story short, I had to go virtually all the way through the archive, until I got to correspondence with authors whose names start with S. And there Einstein had a correspondence with Schlick, who wrote Space and Time in Contemporary Physics. It first appeared as an article (n German) in Naturwissenschaften, which was then something like the German version of Scientific American. Einstein was sent a copy of the earliest version. It had the wrong version of the principle of equivalence in it. And so Einstein wrote back to Schlick, saying, “This bit’s wrong. You've got the wrong principle.” And then he explained why it was wrong. And that letter lays out why Einstein had his different view, and what the different view was. And that for me was the critical thing.

I didn't find that letter for maybe a year. Because when I got to three-double-oh-six, something interesting happened. Now, I've been a little bit self-effacing about my dissertation, because it contains a major omission. In 1913, Einstein started working with Marcel Grossmann, a friend from his university days, and a mathematician. And it’s from Grossmann that he learned about the absolute differential calculus now called tensor calculus, tensor analysis. In the space of less than a year, the two of them together produced a complete first draft of the general theory of relativity. Einstein wrote the physical part; Grossmann wrote the mathematical part. Now, this is 1913, so it’s early. It’s all there.

The whole structure of the general theory of relativity is there, excepting the gravitational field equations. Einstein writes down something that’s pretty close to those key equations. Grossmann reports it, and then says, “Ah, but it doesn't work. You get the wrong Newtonian limit out of this for weak static fields.” And then Einstein goes ahead and publishes the wrong set of equations. He has a derivation for them, but they’re not generally covariant. It’s just wrong, to modern minds. That paper is published. And then things move on a little further. Einstein comes up with arguments for why generally covariant equations would be inadmissible. The best known is the “hole argument.” So this is really a disaster in retrospect. General covariance soon became and is now so essential to general relativity that it is hard to conceive the theory without it. To get a sense of the gravity of Einstein’s rejection of it, it’s a bit like this. Imagine Newton publishing Principia, but when it comes to the spot where he had the inverse square law of gravity, he tells you, “Well, I thought about an inverse square law. It doesn't work. Let’s use the inverse cube law.” Then he goes ahead and publishes the whole thing.

Of course nothing quite works. Everything is a bit off. And then Newton continues to argue that an inverse square law couldn't work. Here gives us a general argument that says that an inverse square law is wrong physically. In the analogy, that is what Einstein did. In retrospect, it was huge mess. What happened? Now, the key thing was that Einstein thought that something pretty close to the modern equations—the vanishing of the Ricci tensor, to be precise—did not reduce to the Newtonian equations in the appropriate circumstances. And that would be a disaster. If you can’t get Newton’s law out of the more general law, you clearly don’t have the right theory. When you're dealing with weak gravitational fields such as surround the Earth from the sun, Newton’s law of gravity works very, very well. You have to recover that.

Anyway, so what happened? Why did Einstein and Grossman think that the later theory could not give back Newtonian theory when it should. Well, we didn't really know. John Stachel had come the closest to figuring it out. When I was in Australia, he had sent me papers, in which he had started to point out what some of the difficulties were. Getting those papers were for me a revelation. Perhaps I can say again what it is about Stachel that I found so important. I had read a lot of history of general relativity. What the existing literature tended to do was to report the easy stuff. “Einstein did this, and did this, and did this, and did this.” And then when something puzzling would happen, they'd gloss over it, and move on to the next easy point. It was sort of chronological reporting, all done accurately and all very learned, but somehow dissatisfying.

There were puzzling moments left unexplained where you really wanted to know what happened. So why is it that he thought the correct equations don’t work? What is he thinking? He’s a smart guy. What is he thinking? And Stachel had pointed out a difficulty. The sorts of geometric structures you get out of the principle of equivalence are actually incompatible, in the way that Einstein worked, with the final field equations. So Stachel pointed out that there was an inherent tension there, but exactly how that worked, we didn’t know. What impressed me was that Stachel had focused in on the thing that was most puzzling, and put serious effort into trying to make sense of it, and had come up with a proposal that did a lot. I began to understand why Einstein had turned away from general covariance.

There was a tension between the way Einstein used the principle of equivalence and what the final equations would be. The principle of equivalence, the way Einstein used it would give you, in many cases, that the spatial slices of spacetime are flat, Euclidian, whereas we know from even simple things like the Schwarzschild solution that the spatial slices deviate from Euclidian. There was a tension. And I found that energizing, because I found that here was someone who was doing history the way that it should be done. I want more than a mere catalog of the steps that Einstein went through in his discovery of general relativity. I want to understand how he did it, as much as possible. And you can see, from everything I said, it’s always devoted towards saying, “Yes, he had a definite research agenda. Without question, far smarter than me. Used in a way that I never would. Moments of insight that I could never reproduce.

But the overall operation was intelligible and understandable.” And that for me is important. I want to understand that. Maybe it’s the epistemologist in me, because I have philosophical interests. I want to know how we find out about the world. Traditional philosophers’ epistemology worries about things like, “If I sense green, how do I know there’s grass?” And I found that a rather uninteresting problem. The more interesting epistemological problem is, “I see bodies fall in such and such a way; how do I know that the gravity responsible is a curvature of spacetime?” Now, that’s an interesting epistemological problem. That’s a problem worth working on.

Zierler:

John, I've asked you other questions where the basis of it was an assumption of larger, grander plans on your part, in terms of career. And maybe I'll get the same response, but I can’t help but ask, in working in the Einstein Papers at Princeton, did you come away from that thinking you can spend your whole career on Einstein? Or were you looking at that as sort of a jumping-off point for larger issues in physics and philosophy and history?

Norton:

No, I had become very focused on Einstein. Let me tell you the thing that happened next, because you'll get a sense of how this would take over my research career. I mentioned that I was working through the papers, and I came to document three-double-oh-six, which is a small notebook that Helen Dukas had marked as teaching notes for relativity. Now, remember, I'm completely steeped in Einstein’s work from 1907 to 1915. I know his distinctive notations. He changed his notation in 1914. So I can just glance at an equation and tell you when it was written. I opened the notebook. I could see that its content was written in the crucial period of 1913. As I flipped through, there was suddenly this amazing page. At the top, Einstein had written down the huge expression for the Riemann curvature tensor. And the words next to it were “Grossmann Tensor vierter Mannigfaltigkeit,” which is “Grossmann fourth rank tensor”. So instantly I knew, OK, Grossmann has come to Einstein, and he has said, “This is the Ricci calculus.” Ricci-Levi-Civita calculus. “This is the major structure that you need to work with.” We know independently that happened.

Here it is happening on the page right in front of me. The expression is written out very neatly as if Einstein was transcribing it. Then Einstein does a calculation. It looks like a contraction that would form the Ricci tensor, the core expression of the later generally covariant gravitational field equations. And at the bottom, there are four second derivative terms that come out of the Ricci tensor. To recover the Newtonian form, you need to have three of them going to zero. Einstein had written those three terms in first-order approximation. In German, it said “sollte verschwinden” which means “ought to vanish.” So this was it! This was Einstein recognizing the problem that would derail his whole project. When someone has the high point of their life, do they know that it has happened? I didn't recognize it at the time, but that was the high point of my life.

There I was realizing that I was right in the middle of Einstein doing the most puzzling thing that he would do in his entire life—to turn away from the correct gravitational field equations. At that point, I did realize something important was in front of me. “Oh, boy,” I thought, [laugh] “This is it—this is really something.” This notebook is what is now known in the literature as the Zurich notebook. I remember John Stachel was drifting past at that moment. I called him over and I said, “John, John, John, look at this! Look at this!” And of course he said, “Oh, wow!” He was very interested. [laugh] And then he wanted to turn the page and see what happens next. And I said, “No, no, no. I'm doing this. Go away!” [laugh] So that became my project—reconstructing that notebook.

It turns out that the notebook gives you a step-by-step account of Einstein’s discovery process, all the way from the first moment when he’s writing down a metric tensor for gravity, through efforts to gradually piece together gravitational field equations, to the eventual decision to adopt the equations that we now know are wrong, multiple efforts to get around the problems that he discovered and then finally a derivation of the incorrect gravitational field equations. It’s all there in this notebook. It’s this incredible catalog of Einstein thinking through the problem step by step. That project became my life. I sat there and day by day, I reconstructed page after page after page. I developed an admiration for Einstein as a mathematical accountant. He could take very large expressions and go down line by line and line, not making mistakes, all the way through. Sometimes he would make a mistake and drop a factor, and he’d go back and correct it. But every single step was there. The moment I realized just how close I was getting to his thinking was this.

Einstein would do a calculation—there’s one line, then there’s the next line, and I couldn't see how to go from this line to this line. How did he do that? So I pulled out a scrap bit of paper, and I’d do a little sum, on the side. Then I take the notebook, and I flip the page, and there’s exactly the same calculation. So with experiences like that I've figured out I knew what he was doing. These are all private notes, by the way. There’s very little written comments. But I learned Einstein’s private code as to how he would mark things to be contracted and so on. Eventually, this became a huge project. And eventually I worked with people in Berlin on this for a long time. We produced a huge four volume tome. We got to know Einstein’s working very, very closely. I remember one of the big debates we had was that in the top right-hand corner of some of the pages, there was a squiggle that looked like this. It sort of goes backwards and forwards. It’s a bit like a caduceus, as they call it, like the snake that Hermes has. So it’s got that kind of shape.

And the question was, what was this about? Was this a special notation for different pages that he would come back to. This was his coded marking? Here I think I got it. I was the only one who had recently used fountain pens. And if you use a fountain pen, you know they don’t always start writing, right? You put the pen to the paper, but ink doesn't come out, right? [laugh] And so you do a little squiggle [laugh] to get the ink flowing. So I tell that story to give you a sense of just how detailed our understanding became of the step-by-step by step processes that Einstein was going through.

Zierler:

How did you formulate a writing agenda? The danger here is that you're just so immersed in the details page by page. How sensitive are you to there’s a larger responsibility to just get this information out in a digestible way to lots of people who would care about these issues?

Norton:

I had no idea at all. Look, you've got to remember, I had finished a PhD in Australia in complete isolation. My exposure to the intellectual community was to talk to John Stachel who was infinitely wise and knew everything. I read the historical papers on general relativity. They tended to be written by physicists for other physicists, so they assumed extensive knowledge on the part of the readership. I was just intent on writing something for an audience of very smart physicists who had some sense of the history of relativity already. Now, I eventually wrote a long paper—it’s “How Einstein Found His Field Equations,” which got published in Historical Studies in the Physical Sciences. And it just went through Einstein’s discovery step by step. It was a very densely written paper, because I had spent a lot of time reconstructing Einstein’s calculations. It’s the sort of thing where a reader might find a chance remark in a footnote, but it took me a week of solid work to come up with that footnote. Why did Einstein do this?

Here’s a quick explanation in the footnote. But it would take a week of intense work just to get that footnote. So yes, this caused me a problem. I wrote that paper, and I sent it off to Historical Studies in the Physical Sciences, largely on the strength that their Volume 7 had been a really solid piece of history of relativity, special relativity. I figured this was the place to send it. In retrospect, it should have gone to the Archive for History of Exact Sciences, but that’s, hindsight.. And nothing happened, right? I'm sitting there in Princeton thinking, “Well, I guess they all know this.” [laugh] Right? It just sat there, and it just sat there. I went to a conference—it must have been the History of Science Society meeting—I think it was in Philadelphia—in 1982. And I ran into Paul Forman, one of the journal’s editors. And Paul Forman chatted with me, and we had a conversation. He clearly had a purpose. And then he finally said, “You know why it’s taking so long for them to deal with your paper? It’s because they can’t typeset the equations.”

Now, this paper was full of big general relativity expressions. These are huge expressions. And this was before the era of electronic computers being used in the way they are now. He said, “They're having trouble. They can’t typeset the equations.” I didn't know what to do. I said, “Look, I've got a problem here. I'll be looking for a job, and I need publications. And this is my major publication. I need to—” And so Paul, he was very kind. He said, “Well, you have to make—” I don’t remember how he worded it—something like “make an offer” or “make a statement.” But I figured out instantly what he meant. And I said, “Well, if you guys don’t want it, I'll send it to the Archive for History of Exact Sciences.” And he said, “OK, good. Leave it with me.” [laugh] Then the paper was accepted. They had real trouble. To give you an indication of some of the trouble, you know what a fraktur font is? The German fraktur font?

Zierler:

No.

Norton:

Think of old Gothic writings. Think of Goethe and Faust and the like. Think of the old letters they use. As we nowadays jump between Latin and Greek letters in our mathematics, in the German tradition, they would use Gothic letters—fraktur—as a way of encoding different quantities. And the journal couldn't typeset Fraktur. They didn't have the font. So I eventually I went to the Chicago Manual of Style, photocopied the alphabet, and mailed it to them, so they could cut out the letters [laugh] and stick them in. It was how these things were done before the modern era of all-computer typesetting.

Zierler:

How did you opportunity at Pittsburgh first come about?

Norton:

Well, I was in Princeton working on the Einstein Papers and working on the Einstein Papers, and my wife was finishing up and finishing up. And I had no real understanding of how the job market worked, or how to get a job or anything. And so I just hung on and hung on. I had applied for a few jobs. I obviously didn't look like [laugh] anything anyone wanted to employ. So I just drifted about. And eventually John Stachel realized I wasn’t going away. I actually thought it would be great to have a job as an editor on the Einstein Papers. You know, I was happy doing that sort of work. I’d love to do more of it. John wasn’t interested. He was hiring other people, but he wasn’t hiring me. So eventually he realized he needed to do something to move me along. He knew Clark Glymour. I mentioned Clark already once before. He knew Clark Glymour at Pittsburgh. And he laid out the problem to Clark.

Now Clark shoots from the hip. Clark was, at the time, the acting chair of the Department of HPS. And he said, “Well, OK, we'll bring you over to the Center for Philosophy of Science.” So I got an invitation to go there. The Department of HPS had had one of their graduate students on the job market. They had funding for him on the assumption he wouldn't get a job, but he did get a job, so they had some spare money. They spent it on me. So I went to the Center for Philosophy of Science as a fellow, supported financially by the Department of HPS. That was my second transformative experience, by the way. I really felt I’d understood how history works from Stachel. But then at the Center for Philosophy of Science, it was an extraordinary year for philosophy of space and time. The office next to me had David Malament. I don’t know if that name rings a bell.

Zierler:

No.

Norton:

He’s a senior sage of philosophy of space and time. Roberto Torretti who had just finished his book on relativity and geometry was there, and a bunch of other people. They included Richard Healey, who worked in foundations of quantum mechanics. It was extraordinary. And that’s where I started to learn how to do philosophy of science, by hanging out with these guys.

Zierler:

Was your sense that the program at Pittsburgh was in growth mode at that point? Was it already pretty well established?

Norton:

The Department had been founded in the early ‘70s. It was an amalgam of a program that had spanned the philosophy department and the history department. I think those were the two departments. Programs that span two departments are not stable and not happy. And so it was turned into a proper department at that point. It was very much an appendage of the philosophy department at the time. The real philosophy of science strength resided in the philosophy department, although HPS was doing well. We had good faculty. But we didn't really have the sort of prominence that I think the Department now has. The Department is now very prominent. The Department has strengthened over the years and became more and more an entity in its own right.

Zierler:

In what ways did your transfer over to Pittsburgh open up new opportunities in terms of your research and your writing?

Norton:

Well, when I was in the Center, I just kept working on the same projects that I had been working before. I wrote the paper on the principle of equivalence. It took me a long time to write that. They had me doing a little bit of part-time teaching. I think I did some undergraduate teaching and a graduate seminar and that was helpful. It prepared me for when I started teaching graduate seminars in history of quantum mechanics. And then at the end of that year, you should move on. You were a visitor in the Center for only a year. I had no real idea what I would do next. My wife was pregnant with our first child. It was a hugely worrisome and stressful time.

What saved me was that the Department had been trying to hire Mike Friedman, and he had been messing them around, and then he turned them down. The Department were worried that the position would be closed down if they didn’t get someone to fill it quickly. And I was there, and on the spot, and so they just offered me the job. And that was terrific. I found myself in this new department, a department that had tremendous strengths in philosophy of science, more so than in history of science. And so I started teaching philosophy of science. Then I realized that I really didn't know what I was doing in philosophy of science. So I started attending other people’s seminars just to learn. That’s where I really learned how to do philosophy of science. I never really got the idea when I was back in Australia. So my early years in Pittsburgh were a very intense learning experience.

Zierler:

What are some of the useful things to understand, for our broader audience, in terms of parsing out history of science versus philosophy of science? Because in a broadly conceived way, there must be a lot of overlap. In what ways are they different?

Norton:

Well, they're different subjects. History of science, as I understand it, is simply trying to find out what happened. Philosophy of science is trying to get a deeper understanding of the foundations of science. So they are in principle two quite distinct things, but they intersect in many, many ways. The clearest intersection, the--as it were--golden years of history and philosophy of science, would have been the Kuhn-Popper-Lakatos era. That’s when philosophy of science and history of science come together. Historians of science who are writing history of science without any knowledge of philosophy are likely in the grip of a very bad philosophy. You can’t write about a science without having some understanding of what you think the thing is, and what the important themes in it are.

For people in the modern tradition, the simplest way of seeing that is to recall an earlier tradition of strictly empiricist writing in history of science, where all that mattered were the experiments. So you give detailed, lengthy narratives of the experiments, and the theoretical ideas are some kind of a superstructure that people work out to fit them. What you think the fundamentals of science are really does matter a lot. If on the other extreme you're a social constructivist about the nature of science, then you don’t care too much about what the experiments were, what the evidence was, what the reasoning was. What’s more important is what are the social structures in which the scientists are embedded and how those social structures motivate them towards certain sorts of results. There are many intermediate cases in between that. But if you don’t realize that you are working with a particular conception of the nature of science when you write the history, you're in trouble, because you don’t understand what it is that you're writing about.

Necessarily, history is selective. You can’t tell everything. Things are happening every minute of every hour of every day of every year. There’s no way you can possibly record everything. I think of the Tristram Shandy problem. (He’s a fictional character who tries to write his complete autobiography. He takes one year to write up the first day of his life, one year more to write up the second day, and so on.) You have to select out what matters. And so you must have an idea of what it is that matters. And that’s coming from philosophy. So that’s my speech to historians—why you need philosophy. It goes the other way too. Philosophers of science love to opine about the nature of science and how science works. If you are doing that, you better know what actually happened! Because things don’t go the way that you would think.

One of the enthralling things in history of science for me is always to discover that things aren’t the way we thought. Here’s just a trivial example, just off the top of my head, because I ran into it recently and I thought it was interesting. You know about Penzias and Wilson and the cosmic background radiation? It’s very easy to imagine—“Well, that settles it. Big Bang cosmology takes the day.” Steady state theory is gone. Right? That’s an oversimplified gloss, and you'll find it in various places in various textbooks. At the crudest level of description, it’s correct. It’s sometimes used as an example of inference to the best explanation in philosophy of science. This is a form of inductive inference that philosophers identify. The Big Bang is the best explanation of the cosmic background radiation, so, according to inference to the best explanation, we should infer to it. But that isn’t what happened. The cosmic background radiation was discovered. That it was thermal could not be known. Because to know that a distribution is thermal, you need to measure it at many different frequencies. Penzias and Wilson weren’t measuring at many different frequencies. Might this be the dissipating radiation cloud of a Big Bang? It wasn’t even talked about that way.

It was the—I'm trying to remember—I think the expression that was used at the time was “primeval fireball.” Remember, “Big Bang” was a term of disparagement that Fred Hoyle, the steady state theorist, thought up. It took decades of gradually working through all of the possibilities until eventually it became very clear that this is incontrovertible evidence of the Big Bang. It was really I think the COBE data that did it. It was only then that things were really nailed down. Because then you have convincing data—you've seen the graph. The COBE data gives you this perfectly beautiful black-body curve. But if you just go on the superficial history or philosophy, you say, “What’s a striking example of inference to the best explanation? Well, cosmic background radiation. The physicists immediately inferred that the best explanation is the Big Bang. Steady state theory was dead.” No, that didn't happen. And if that misreading of the history is important to your philosophy, you're in trouble. Once you realize that, you have to come up with a different account of the canonical cases of inference to the best explanation. And I have. It’s two chapters of my big book on the material theory of induction (now in full first draft on my website).

Zierler:

I asked the question precisely because I was hoping that you would lay it out that way. What you're saying is that both disciplines really do need each other.

Norton:

Yes.

Zierler:

At the individual level.

Norton:

Yes. I work in history and philosophy of science, and it’s a little hard to know what history and philosophy of science is now. Once upon a time, you could say, “Oh, it’s the sort of stuff that Popper and Lakatos do.” Because it’s very clear how the two fit together. Now, I work differently. My primary interests are philosophical. So I'm interested in the nature of inductive inference, how evidence works, and I've developed various approaches to it. But what’s distinctive about the work that I do is that I do large numbers of case studies. The case studies aren’t merely illustrations of foregone conclusions. The case studies are the origin of the ideas. So, for example, I got very interested in the nature of explanations in science.

I thought the modern accounts of it were terrible, because they had lost real science. They had stopped talking about real examples in science. I went back to the canonical examples and just worked them from scratch. There’s a whole bunch of cases that are standard, canonical examples of inference to the best explanation. And I came up with a completely different account. In fact, it’s got nothing to do with explanation. There’s no cogent idea of explanation that has any special inductive powers. That’s one of the conclusions you come to. And there’s a whole story. I won’t go into that, because it’s irrelevant to present purposes, but it’s a way of working. It’s a way of integrating philosophy of science and history of science, in which you really take the history seriously.

Zierler:

John, you mentioned case studies. I'm curious if you have studied the way that certain subfields have ebbed and flowed over the course of the 20th century. I'm thinking specifically of—many people I've talked to say in the fifties and the sixties, general relativity had fallen out of fashion. I'm always struck by how a scientific concept could be—a subfield of physics can be—what’s the word?—it could be a part of such social constructions.

Norton:

Gets marginalized.

Zierler:

Have you spent any time thinking about that?

Norton:

I haven't. But if you really wanted to read about this, Jean Eisenstaedt is the guy who has lengthy analyses of this. He has one paper with the title “The Low-Water Mark…” I can tell you roughly what happened. So general relativity hits in 1915, 1916, and so on, and it’s clearly an extraordinary theory and there’s a huge amount of energy and interest in the theory. Then there’s the eclipse expedition of 1920. Everyone is again very excited. Einstein becomes a popular—sorry, it’s 1919, but it’s more in the 1920s that the excitement really takes off. Einstein becomes a popular figure, and everyone wants to know about relativity.

Then quantum mechanics hits. Now, it’s not clear how you advance general relativity, because the distinctly relativistic effects are in very exotic domains of intense gravity. There were gravitational waves, but there was no realistic possibility of detecting them in the 1920s. Quantum mechanics hits, and it is this incredible, endless repository of problems. It’s not just the matrix mechanics of Heisenberg, the wave mechanics of Schrödinger and putting the two together. That’s just the first step. Then you're moving on to quantum electrodynamics. And then you're going into the quantum mechanics that underlies nuclear physics. And there’s just enormous amounts of work to do, and people get swept up in that.

Meanwhile Einstein is skeptical of quantum theory, and he’s pulling himself away from it. He doesn't work on it. So Einstein, who would be the core figure in general relativity, is losing interest in where the mainstream of physics is going. Then, of course in the 1930s, everything got shot to hell with the coming of World War II and the dismantling of the German academic environment by the Nazis. In World War II, we have the Manhattan Project, MIT Radiation Lab. All of this stuff—this is all electrodynamics, and especially quantum mechanics and nuclear physics. And there’s just so much energy there. But contrast, what are you getting out of general relativity? There’s very little novel coming out of general relativity. It becomes the plaything of mathematicians. The mathematics is interesting, but there’s really not an enormous amount of exciting physics to be done. Whereas nuclear physics--it’s taking off. The standard model hasn’t happened yet. The pieces are there. You've got a generation of physicists who have worked at the Manhattan project. They're experts in neutrons and neutron cross-sections and things like it. Feynman diagrams are coming out. It’s more a matter of if you're a young physicist and you want a good career, where do you go? Is anyone going to tell you, “General relativity, that’s the future…” Why would someone advise you to do general relativity when there’s all of this excitement in quantum mechanics?

Now, what changes things is the novel interest of mathematicians, people like Roger Penrose, Bob Geroch, Bob Wald. Maybe the dates are a little bit askew. And of course Stephen Hawking, George Ellis. And what they start to do is to show that there are these interesting mathematical results, singularity theorems and the like. And so they do two things. They bring a new level of mathematical rigor to general relativity. The textbooks take a turn with these people. And also it becomes clear now that there are exciting things to be explored in the mathematics itself that are really meaningful to physicists. We're thinking about Big Bangs. Is the Big Bang singularity inevitable? What are the chances of a near-miss from a collapsing, earlier universe spreading out again? When gravitational collapse happens, is a singularity inevitable? Are black holes really pathologies of spacetime? These things then start to become interesting and important. And of course there’s always work on gravity. So I don’t know; that’s my view of it. I've not researched it directly. It’s just what I’ve picked up from being around others who work on this. But that’s my impression of what happened. Jean Eisenstaedt is the guy to read. He would nail this down.

Zierler:

In your research on Einstein, I wonder if you can comment on the extent to which he was interested in where his ideas would head long after he had passed. Did he spend much time thinking about that?

Norton:

I don’t know of any evidence that this was a concern for him. He tended to write reactively. Someone who has an interest in their legacy is going to be producing definitive works that will be the thing that you would consult forevermore. He gave some lectures. His only textbook on relativity, the Meaning of Relativity, was written as a part of the lectures he gave in Princeton in I think 1921. They're fairly short. There’s not a lot there.

Zierler:

Perhaps a different way of asking that question is if you got a sense that he thought that any of his particular works would age better than others.

Norton:

Well, he was completely convinced that what we now call the geometric approach that he took in general relativity was the right way to go. That was the future, he thought. I should mention, we now think of general relativity as geometrizing gravity. He denied that, and with good reasons. My colleague Dennis Lehmkuhl has written nice papers on this. But anyway, that idea of joining together the geometrical properties of space and time, gravitational processes, and electric and magnetic processes and whatever else might be there, into one single unified structure, that for him was the core of his research agenda starting in the early 1920s and maybe even earlier, onwards. He spent all of his time on that. That was the thing he was focused on. I think he expected that in the long run, that would be the way that we would end up with a unified physics that would bring together all the various forces of nature. Certainly he never wavered from that.

Zierler:

Did he spend a lot of time, as far as you can tell, thinking about how advances in technology and advances in experimentation might prove or disprove some of his theories?

Norton:

He was very astute on that in the early years. So typically a standard format for him for a paper would be he’d lay out a novel piece of theory, and then he would finish with a number of experimental tests, typically three. So the special relativity paper ends that way. The light quantum paper ends that way, the photoelectric effect being one of those experimental tests. The general relativity review article of 1916, ends that way. It’s clear that he was well-read in the experimental literature. So for example, he could pluck out results on the photoelectric effect of importance to his light quantum just because he knew that literature fairly broadly. In his early years he was patent examiner in the Bern patent office, so he knew the practicalities of invention. His family had run an electrotechnical firm in the 19th century, and he maintained a side business as an expert witness in patent lawsuits. And we have some documentation from those activities. I've not worked too much on his patent interests.

József Illy recently wrote a book on this. It catalogs what we know about Einstein’s patent interests. In the later years, he seemed to drift away from the experimental side. He had some flirtations with experimental stuff now and again. There were a few cases where he did work experimentally, but never to any great effect. Later in life, he, with Leo Szilard, designed a refrigerator. They have a joint patent for this refrigerator. I don’t know much of the details. One of the interesting things that came out of historical research by the Berlin group was this. I had noticed at one point in World War I that Einstein wrote a paper for Naturwissenschaften on how airplanes fly. And he explained rather beautifully and simply how the Bernoulli effect works to produce lift for a wing. Why this sudden interest in wings? At the same time, he was designing an airfoil, and it was actually tested in a wind tunnel. We have the tests that were undertaken in the wind tunnel. I've looked at the tests. I haven't really studied them.

I was shown the tests by the people in Berlin who were working on them. I could use my earlier experience in chemical engineering. Fluid flow is one of the core topics we deal with. And just from what little I know, it looked to me that the wing design was not especially good, because it looked like it had very poor stall characteristics. When the angle of attack of a wing increases, you lose the streamlined flow of air over the wing’s surface, and eventually you get turbulent flow, which means the wing loses its lift and the plane stalls. Well, what you don’t want is a sudden onset of turbulence, because then you suddenly lose lift, and that’s bad. What you want is a clear warning that this is happening. If you’re a pilot and you start to approach stall, you know because everything starts to shake and shudder, because of turbulence. And my hasty scan of it was that it did not have good stall characteristics. It is no shame to him, because the streamline to turbulent flow transition is a remarkably intractable one to analyze. If I remember correctly, this was Heisenberg’s dissertation, as a way of proving to one of his professors that he could do work that was classical.

Zierler:

You mentioned before that Einstein was not terribly distinguished as a philosopher himself. Is your sense that he did have a pretty good idea of the philosophical implications that his thoughts on space and time would have in more capable hands?

Norton:

He certainly knew philosophers who worked on these implications. The premiere expositor of philosophy of space and time after Einstein’s work was Hans Reichenbach. And Hans Reichenbach was in Berlin and took lectures with Einstein on relativity. They knew each other. We have the lecture notes, by the way, that Reichenbach took, in our archive in Pittsburgh. So Einstein certainly knew of this work. What’s noticeable is he doesn't say much about it. I think the view we have now is that Reichenbach’s interpretation was excessive. He pushed things too far. Reichenbach is a conventionalist. Einstein writes about conventionalist themes in a famous paper, “Geometry and Experience.” I think it’s very different from the Reichenbach tradition. When you read the two papers side by side—when you read Reichenbach’s work with Einstein’s papers side by side and you pay close attention to what they're saying--I think they're saying different things. But Einstein does not correct Reichenbach. I found the same reluctance to correct with the principle of equivalence. Einstein corrected Schlick once on the principle of equivalence. Everybody thereafter continues to use a different version, and Einstein says nothing more about it.

Zierler:

Slightly different topic—what are supertasks and when did you get interested in them?

Norton:

Oh. Supertasks arise when one completes an infinity of tasks in a finite time. I got interested in them just because it has been a long-term side topic in philosophy of science. I can give you a bit of the history of how these things came about.

Zierler:

Please.

Norton:

They arise from the finitist movement in mathematics. The movement says that infinities literally make no sense. You’d never have an actual infinity. What does it mean to say that there are infinitely many numbers with such and such properties? For example, we can enumerate prime numbers: two is the first prime, three is the second prime, five is the third prime, and so on up. So that’s a mapping that goes from counting numbers—one, two, three, four five—to prime numbers. If you're unintimidated by infinities, you would just say, “Oh, there’s an infinite structure there.” Right? For every counting number, there’s a prime number. But if you're a finitist, you've gone one step too far. All you can say is that for any number, I can pick a prime number, and for the next number, I can pick another prime, and for the next number, I can pick another prime. But now to say that there’s an actual infinity is an extra step that you should not take. I know finitism sounds odd, but that seems to be the run of it.

Hermann Weyl, the very distinguished mathematician, wrote Space, Time, Matter, one of the most important early textbooks in general relativity. He had these finitist leanings. He remarks that you cannot affirm the truth of an infinite statement like that, because to affirm the infinite truth of a statement like that, you would need to check that the number assigned to one is a prime, the number assigned to two is a prime, the number assigned to three is a prime. And so on. You would need to complete an infinity of affirmations, which of course you cannot do. So people take this up and say, “Well, yes, of course you can. You just do them faster and faster.” Adolf Grünbaum, a professor here at Pittsburgh for the longest time, who died fairly recently—that was one of the things that he was writing about. “Yes, of course you can do that. There’s no logical problem in completing an infinity of acts.” So one of the things philosophers like to do is play around with how you might complete these infinite sets of actions.

I became interested in playing around with infinity-based puzzles of a similar kind. The Thomson lamp is a classic case. You have a lamp that you can switch on, off, on, off, on, off, on, off. And you accelerate the switching, so that by the end—by some finite time--you have switched it infinitely many times. At a minute to midnight you turn it on. At half a minute, you turn it off. At a third of a minute to midnight, you turn it on, and so on. The question then becomes, what is the lamp state at midnight and thereafter? Is it on or off? Well, it can’t be on, because every time you turned it on, there’s a later time at which you turned it off, and, similarly, it cannot be off. So what do you make of things like this? I regard these as simply pedagogically useful lessons in philosophy. They force you to think clearly. There actually is no profound problem here, but in order to demonstrate that, you need to think clearly in the way that philosophers have to think. And so the exercise of thinking that through then becomes a huge amount of fun. I think the most interesting result that comes out of it is the following. You know the theory of Turing computability?

Zierler:

No.

Norton:

There are certain tasks that are in principle incomputable. No computer can do them. The halting problem is a classic example. Say that you have to write a program such that if I give you any other program in that computer language, you can tell me whether that second program will crash when run on some input. Now, it sounds like it might be really hard to write the program, but there doesn't seem to be any barrier in principle for doing it. It turns out there is. In principle cannot be done. You end up with internal contradictions if you try and prove such a crash testing program exists. Now, one of the premises of this impossibility is that a computer can’t actually complete an infinity of programmed steps. While there is no logical barrier, in physical practice, you actually can’t do it. I can’t build a computing machine that will actually complete an infinity of actions. That is, the result depends on what is physically possible as opposed to restrictions from logic. What the supertask literature shows is that there are odd space-times in which you can complete an infinity of actions. In them, in one part, you have a timelike worldline, the world line of the calculator, that has infinite time elapsed along it, and that timelike worldline lives entirely in the causal past or another event. An observer at that event can see the completed infinity of calculations of the calculator.

Zierler:

When you say “odd”—in what context? What does “odd” mean here?

Norton:

Oh, it means that you'd have to go to spacetimes with singularities. So one of the ways of getting to a space-time like this is you need to find one that has a black hole of a particular type. I think it’s a rotating black hole. I can’t remember the exact conditions now. And to be at that point where you see everything, you need to throw yourself into the black hole. So I'm not saying this is practical. The idea is that on the timelike worldline, you would have a calculator calculating some infinite task—infinitely many cases. For example, Fermat’s theorem could be just checked by checking all triples of numbers. Just check them all, one by one, and off you go; you're done. Then the person watching can see this whole thing. Now, this is dramatically impractical for all sorts of reasons. But what it demonstrates is that there’s a cogent physical theory in which completing an infinity of computational steps can happen. So the premises of proofs for the uncomputability results in computation theory have a physical basis. They aren’t purely logical results. They have a physical basis, which includes the claim that an infinity of actions cannot be completed.

Zierler:

Another topic that I know you've spent a good deal of time on and I wish you could explain a little bit about—what is Maxwell’s demon, and what is its relationship to the thermodynamics of computation?

Norton:

Oh, dear.

Zierler:

[laugh]

Norton:

Oh, this is a huge topic. What is Maxwell’s demon? Well, here’s a case where the history is fascinating. There’s a historical story that’s so interesting, so I'll go through the history. That’s the best way to see it. In the middle of the 19th century, the realization came through that thermal processes were underpinned by molecular processes. Why does a gas expand? A gas expands because it consists of many, rapidly moving molecules, and if you remove a barrier confining the gas, these molecules shoot out. Macroscopically, you just see the gas expanding, like when you puncture a tire and the air comes out. So, how do you handle this idea that gas processes are all really just the mechanics of particles? Well, the natural thing would be to do the Newtonian physics of a collections of particles. You set up a phase space. It’s going to be a phase space with dimensions of something like ten to the power of 24. You set up Newton’s equations in Hamiltonian form. And then you start solving these equations to try and figure out what the particular behavior is going to be.

You can see pretty quickly that solving Hamilton’s equations for a particular gas will be very hard. You need to specify ten to the 24 (roughly) initial conditions, and then grind through a calculation, which is just not practical. So what do you do? Well, you do what we now do. You approach the whole thing statistically. Now, the idea that it’s appropriate to give a statistical analysis of a mechanical process was novel, and this is what Maxwell was concerned with. He was trying to introduce his colleagues to the idea that it’s OK to use statistical techniques on mechanical systems like this. One of the things he was concerned to do was to explain what the status of the second law of thermodynamics is. How does the second law of thermodynamics come about? His point was that if you could actually see and manipulate individual molecules, there would be no second law of thermodynamics. The second law of thermodynamics is an artifact of the statistical description of the gas.

If you coarsen your description of things in the world such that you can only talk about averages and distributions and the like, then a second law of thermodynamics appears. If you are working at the level of individual molecules, then there is no second law of thermodynamics. What’s a simple proof of its absence? Well, he says, imagine a chamber with a gas. There’s a wall separating the two sides of the gas and a door in the wall. There is a being, the demon, who can see and react to individual molecules. Because this being can work at the level of individual molecules, the being can open and close the door in the partition that separates the chamber into two parts such that all the fast-moving molecules get collected on one side and the slow ones on the other side. We assume that the being requires no expenditure of work to do this sorting.

The outcome is that the side with the faster moving molecules gets hotter and the other side gets colder, without any work being done, in contradiction with the second law. Therefore you can see that if you work at that level of description, there is no second law. Of course we can’t work at that level, so that’s why there’s a second law. This was Maxwell’s point. Then things move along. An important development in the early 20th century is that thermal fluctuation phenomena are affirmed. The idea that you could work with individual molecules doesn't appear, but there is something close to it. The distinctive behavior of individual molecular actions is observed in Brownian motion in particular. Einstein established in 1905 that tiny little particles suspended in fluid are jittering about because they're mobilized by very many, individual molecular collisions. That we now know as Brownian motion. We're getting close to being able to manipulate individual molecules with that.

When one of those particles jumps up, the heat energy of the water is being converted into work that lifts up the particle. And that is a violation of the second law of thermodynamics. Poincaré had already presciently suggested that here we see in miniature the working of Maxwell’s demon. It became a serious project for a while, to try and figure out whether you could actually accumulate these miniature violations of the second law of thermodynamics into a macroscopic violation. And there were serious proposals for doing it. Smoluchowski produced what I regard as the definitive refutation. Smoluchowski was able to argue that any attempt to accumulate these little fluctuations would itself be defeated by fluctuations. The Smoluchowski trap door is famous here. Feynman used Smoluchowski’s ratchet and pawl construction to show that the accumulation will fail. That’s Smoluchowski insight again. But there was a loophole. What if you could operate some of these machines intelligently? In other words, do what Maxwell had precluded as a possibility. Might that enable you to violate the second law of thermodynamics?

What happens now is that there’s a change of viewpoint. People start to regard the second law as sacrosanct, and they want to argue that even if you could do that manipulation, you couldn't undo the second law of thermodynamics, at least on average. This is the famous Szilard paper of 1929. He sets up a one-molecule gas in a chamber and imagines how you could reverse the second law of thermodynamics by manipulating it. And then he says, “Uh-oh, this is not good. We have to save the law.” At one point in the cycle, the operator, the demon, has to observe where this particle is. And Szilard postulates that it is the gaining of that information that has an entropy cost attached to it. This now connects entropy and information. This connection protects the second law. It’s simply a speculation on his part. He gets the result by working backwards to the entropy cost needed to save the second law on average.

Anyway, the idea takes off in the 1950s as information theory becomes all the rage in the wake of Shannon wonderful work on the physics of communication. Famous people— most famously, Brillouin-- work on this idea and argue that the Szilard analysis is correct. The gaining of information, whatever exactly that might mean, has a thermodynamic cost and this protects the second law. The arguments look a little shaky to me, but OK, fine, I initially thought. It’s a little odd, but why not. Then in the 1970s, the computational scientists get involved, and they say, “No, no, no. This is all wrong. A demon who gains information can gain the information without doing any thermodynamic harm, without creating thermodynamic entropy. But the demon has to erase his memory afterwards, and it’s this computational process of erasing his memory that is the real thing that saves the second law. Erasure necessarily creates thermodynamic entropy.”

That was just baffling. We are supposed to think that all the experts so far had gotten it wrong? Szilard? Von Neumann? Brillouin? Then it became clear to me that this whole tradition of connecting thermodynamic entropy, information and computation was foundationally very muddled. I've written on this at great length. The argumentation that’s being used here is sloppy to the point of being indefensible. I've laid that out in rather painful length. These arguments just don’t work the way their proponents think they do. But the most important thing for me is that they're all completely unnecessary, because the impossibility of a Maxwell demon was I think first cogently demonstrated by Smoluchowski—in 1912, 1913, thereabouts--without any informational or computational notions.

And then finally, if you look at it again, the impossibility simply follows from the Liouville theorem. There’s a classical derivation of this in which you work in classical phase spaces, which is perfectly straightforward. Because individual molecules are quantum objects, their quantum character needs to be considered. And there’s a quantum mechanical analog of the result. And so the whole digression--into information; the thermodynamic cost of gaining it; no, that’s wrong; there’s a thermodynamic cost in erasing memories—it was all completely unnecessary in analyzing the Maxwell demon. It was impossible from the start, for reasons that had nothing to do with information or computation. But the idea has taken off. People love it. People love to talk about computation and information. In philosophy we value precision of thought and argumentation. Here this literature fails badly. It’s so imprecise and dangerously so. It’s just sloppy thinking over and over again. I'm very tired of it. You can see, I've written a lot of papers on this. They lay out my complaints in some detail.

Zierler:

Another basic question, also something that you've thought deeply about—can you talk a little bit about what you see as some of the most major philosophical implications of quantum theory?

Norton:

[pause]

Zierler:

[laugh]

Norton:

OK, well the major engine of philosophical work in quantum mechanics has of course been the measurement problem. There are many solutions to the measurement problem. None of them has gained universal acceptance. As a result, it’s pretty clear that none of them work. I don’t know that it’s a philosophical issue as much as it’s a physical issue. There is this process that happens when we do a measurement. In other words, it happens all the time. You've got a Geiger counter that goes “click.” That’s a measurement. Because a radioactive atom decays in all directions, it sends out a wave that goes in all directions. You measure it and the particle is at one place only. Using the old collapse of the wave packet story, you've collapsed the wave instantly down to a point. We don’t understand what is going on there. There are many different accounts of it. It’s an incompleteness in the theory. We hope that eventually someone will be able to figure it out. OK, so that’s the big one that keeps coming back over and over again.

The next thing that is really interesting and really important is non-locality. Again, you surely know this. In principle, if you get two particles, entangle them, and separate them, they remain entangled forevermore unless something else interacts with them. So they can be as far apart as Alpha Centauri and the Earth but still be entangled. That means that a measurement on the particle at Apha Centauri would instantly collapse the wave of the second particle here on Earth as well. This idea that we have a non-locality in space is something that is not recognized by many people who just work in philosophy independently of the science. They work with an ontology that is roughly that of the 17th century. They talk about particles, localized particles, and the properties that they carry. If you take quantum mechanics at all seriously, the world is foundationally nothing like that.

Take a simple thing like the idea of the particles as living within an ordinary three-dimensional space. That is not the quantum picture. Quantum mechanics is done in a configuration space. So if you have a cloud of a thousand particles, in the old conception, you would think of a thousand points moving in a three-dimensional space. Instead, in a configuration space, you have one point moving in something like a 3,000 dimensional space. And does it make a difference? Yes, that makes a difference. It’s the transition from an ordinary space to a configuration space that gives you entanglement.

There are more things. Quantum mechanics in the 1920s was regarded with some horror because it contradicted determinism, the doctrine that the present state fixes a unique future state. And well, so what? Nowadays we say, “Well, big deal, the world’s not deterministic. Some processes can only be said to have probabilistic outcomes, and that’s the end of it.” Well, it was worrying then, because in the 19th century, they decided that determinism was synonymous with causality. To be causal was to be deterministic. And so in the 1920s, they were saying “causation is lost,” and this was a serious problem. But of course what then happened is we realized, “Oh, we need to adjust our ideas about causation. Here’s a way of thinking about causation in which everything’s fine.” This is something I've written on. The original paper is called “Causation as Folk Science.” This quantum episode illustrates what that I've been arguing at greater length. Things in the world are connected in all sorts of interesting ways. The metaphysics of causation has failed completely in telling us anything useful about that. We repeatedly learn through new science that things connect in ways we did not expect. All that happens is that the metaphysicians of causation come along after the new discovery and clean up their last mistaken insistence on how things have to connect.

They keep just adjusting their ideas to whatever the next idea is. They want us to think that they have discovered some fundamental truth about the nature of the world. The world is fundamentally causal, and it is the obligation of science to find the expression of that idea in their individual domains. That isn’t happening at all. The metaphysicians of causation can supply us with no prior idea that has any kind of stability. The latest science is always overturning what their last idea was. Another example is Newton, who thought that action at a distance was so absurd that no one who had competent faculty in philosophy—he meant physics and science—could ever fall into it. Well, that didn't last very long. In no time at all, his own theory was regarded as an action at a distance theory. There’s probably more. If you give me more time, I could go on at greater length. I’ll spare you.

Zierler:

I don’t want to exhaust you with too many of these incredibly broad questions, but I do want to ask one more, in an attempt to do some kind of comprehensive coverage of the big areas in physics and philosophy that you've worked on. Can you explain the debate surrounding the attempt to recover phase transitions in statistical physics? What does that recovery process look like, and what are those debates?

Norton:

I've only published one or two papers on this. It’s not a major—

Zierler:

Oh, good. That’s an easier one for you, then.

Norton:

It’s not a major issue for me. Here’s the way the problem is set up. Phase transitions, when understood at the thermodynamic scale, involve discontinuities in thermodynamics properties. So you look at the phase diagrams; you see kinks, right? Now if you do a component analysis, a molecular analysis, or one using various momentum modes, all of those quantities are derived from a structure known as the partition function. The partition function is an exponential with many, many components. The key thing is that it is an exponential function. And you recover the thermodynamic properties by acting on this partition function with differential operators. This differential operator is going to give you entropy. This differential operator is going to give you energy. This differential operator is going to give you free energy. And so on. Now, the difficulty with an exponential is that it’s an analytic function, and analytic functions cannot have kinks in them. Right? [laugh]

The differential operators applied to the partition functions to recover thermodynamic properties do not create any kinks. So then the question becomes, how can a molecular scale physics give you the discontinuities that are phase transitions that we see? One analysis says, “Easy. Any finite number of components will always give you unkinked curves (that are differentiable in all orders), because they're analytic. But if you take a limiting process and go to the limit of infinitely many particles, then the limit of those functions will have a kink in it.” So therefore, it looks like we need to talk about infinitely many particles, in order to have phase transitions. Now I think you can sense almost instantly the paradoxical character of that. Phase transitions happen all the. time. You drop an ice cube into water and it melts; that’s a phase transition. If we know nothing else from chemistry, we know that there are finitely many molecules in the glass.

So if that phenomenon requires there to be infinitely many molecules in the glass, then we have refuted our basic understanding of matter that has driven chemistry from the 19th century onwards. Avogadro’s number is not finite; it’s infinite. So what are we to do with this? Well, there’s a tremendous amount of chatter going backwards and forwards. My contribution to it has just been to say something that many other people are saying. I hope I've said it more clearly than most and that it’s worth writing a paper on it. It is that you never actually realize an infinite limit in the number of components present in the analysis. You never actually consider infinitely many particles. You just consider a system with many particles, and many more, and many more, and many more, and many more, but never actually look at an infinite system. You do look at how the properties of these always finite number component systems converge in the limit. And they will converge to something that has a kink in it, without you ever actually having an infinite system of particles.

I think that’s the right way to think about it. And then of course there’s a further complication—that there is this sharp kink in the first place is only a coarse description. If you analyze it very, very closely empirically, you'll find there isn’t actually a sharp kink at all, so the whole problem goes away. This problem has generated massive amounts of confusion. One of the simplest confusions is the following. You know the philosophical debates on reductionism? A standard example of reductionism is that thermodynamic systems are just a whole bunch of molecular components. So some people have tried to use this effect as saying, “No, they're not, because the reduction fails.” And that’s just confused thinking, but there’s plenty of confusion out there.

Zierler:

I want to ask now some sort of broadly retrospective questions about your career. Are you very much interested in the idea that if physicists pay more attention to the philosophy of physics, that might make them better physicists? Have you thought about that, or do you care about that? Do you think that that’s true?

Norton:

I really like it when physicists take an interest in philosophy of physics. But it puts a lot of pressure on us. The number of times a physicist comes to talk to us—and what they're looking for is what Einstein was looking for—the great idea that will open the door to the next theory. Well, if I had that great idea [laugh], I’d use it myself. I'm not going to give it to you! I'm going to go and collect my Nobel Prize. So I would like to think that physicists are interested in philosophy of physics for the same reason that I am. It is not my job to try and discover the next theory. In fact, trying to discover the next theory can be in tension with rigorous philosophy, because you don’t want to nip a flower in the bud. If you're too critical about the logical cogency of ideas, as philosophers must be, you can destroy something that might otherwise be useful. I just want to add a footnote here. This is not the case with all the talk about information entropy and computation. They have been at it for so long that we now know they're weeds; they're not flowers in the bud. They've had plenty of time to prove themselves. Szilard’s paper was almost 100 years ago. The founding papers in the thermodynamics of computation are getting on to be about 50 years ago now.

Fifty years is a long time not to produce a cogent result. Let’s set that aside. OK, so getting back to the main line of your question. As a philosopher, I'm interested using our best present physics to answer the question, how is the world? These are the age-old questions of what is the nature of time, what is the nature of space, what is the nature of matter. And I want it to be done precisely and clearly. What does general relativity tell us about space and time? It turns out that we need to be careful. It’s not a general relativity of motion. It’s something else. Careful philosophical thought will give a good answer. What is quantum mechanics telling us about the nature of matter? If you've done any reading in the foundations of quantum mechanics, it’s a mare’s nest of problems.

There’s a huge amount of work to be done determining just what quantum mechanics is telling us about the nature of the world. In particular, we will eventually need some resolution of the measurement problem to get there. I would like to think that this is something that interests physicists. These are interesting and worthwhile problems in their own right. They require us to take seriously our best theories, as they are now, and to try and understand what they're telling us in foundational terms. That said, I'm always happy when physicists find work that we do useful. Sometimes this can happen. One of the examples was the hole argument. Have I mentioned the hole argument that Einstein… ? Yes, I did.

Zierler:

Wait, say it again? Which experiment?

Norton:

“The hole argument,” which was introduced by Einstein in 1913 as a way of arguing that general covariance is physically uninteresting. With a colleague, John Earman, also with inspiration from John Stachel, we retooled it as a modern argument in philosophy of space and time. And that argument has proven to be very useful for people who work in loop quantum gravity. They are trying to debate with string theorists. String theorists set themselves up in such a position that they become prone to the difficulties of the hole argument. So that’s an example of where philosophical work has been useful. I also would like to think that the criticism that I've been mounting of the thermodynamics of computation and related matters has been useful. I feel that it’s a rather hopeless endeavor, because the engine just chugs forward. But I've been told that my criticism has forced the people to become much more careful in what they do and say. And I think that’s a useful contribution. I think it’s as good as it’s going to get. And I'm quite happy to hear that.

Zierler:

You mentioned string theory and so I have to ask now—I'm curious—the criticism of certain physicists on string theory—that string theory has worked itself up to a place where it’s essentially immune from evidence-based criticism—what role do you see and people like you having as a productive place in that debate?

Norton:

Well, I haven't engaged in that debate. The proponent of that is Richard Dawid. He’s a string physicist who turned to philosophy. I'm an empiricist, in the weak sense. That just means that I believe that what we know of the world, we learn through experience of the world. And when our theorizings are disconnected from experiences in the world, they become fantasies that typically drift off into areas where things are just not correct. String theory is in great danger of exactly that happening. We are told it does need to have an empirical basis. Things are different now. On the contrary, I don’t think science is any different now than it was 100 years ago in this aspect. Having a sound empirical foundation for science is simply essential. We can build many fine castles in the air, but they need to be grounded in experience and experiment. That’s all I can say now. And how that applies to string theory; well, what it says about string theory is this. Don’t make any rash decisions until there’s good experimental support for it.

Zierler:

And maybe this question is more about your feelings on academic politics than about the philosophy of it, but when there are certain campaigns by physicists to make departments string-free, so to speak—like that attempt at Harvard in the 1980s—what is your reaction to something like that?

Norton:

Well, this is more a sociology of science question, And in the sociology, I have a view, even though it’s not well researched. It is that we need to distribute our resources. I don’t like to see everybody working on just one avenue. I like to see the resources distributed. And those resources ought to be distributed according to the likely fertility of the program. So if you have a program that has a high probability of success but maybe a low payout; yes, support that well. If you have a project that has a very low probability of success but if it succeeds, it’s a big payoff, yes, by all means, put some resources into it, but don’t bet the farm on it. My sense of what has happened with string theory is that the resource allocation has been skewed. If you think of the distribution of resources in the way that I have, string theory has collected a disproportionate amount of resources in relation to the risk-benefit analysis that I just gave. What is the correct distribution? Here, I don’t know. This is something that I would defer to physicists to figure out for themselves. But I've heard enough from physicists themselves to indicate that some sort of rebalancing is needed. I don’t know what happened in Harvard in the 1980s.

Zierler:

To continue on that thread, to change the question up a little bit, the way that you have identified string theory as perhaps being a certain degree of a misapplication of resources, what are certain subfields that you feel are ripe and do not have a sufficient amount of interest and energy devoted to them?

Norton:

Oh, you need to give me a heads up for a question like that. I can tell you the thing that’s interesting me at the moment. I might get into this; I might not. I'll have to see. What I'm seeing is machine learning entering into almost every area of physics. Of science, rather. And I'm curious what that means methodologically. Is this a methodological shift that philosophers need to take seriously, or is it just more of the same? So you can see some sciences have sprung forth entirely because of computers. Take complexity theory, right? A lot of it comes from our increasing ability to compute massively. We have new ideas that are coming from computer simulation. Are we seeing that with machine learning or not? People in machine learning of course make big promises. Can they deliver on them? Things look promising. I understand that astronomical and astrophysical searches now are being done by machine learning techniques. So we can now search the heavens in a way that ordinary human observers never could. That could be exciting.

Zierler:

One thing we haven't touched on in your career and something that you are interested in is communicating some of your research and your ideas to a non-specialist audience, a broader audience. What kinds of audiences do you try to reach, and what are some of the most important things from the vast amount of research and thinking that you've done that you want to distill for that broader audience?

Norton:

Well, I've done a few things. The most important thing is my website, Einstein for Everyone. I've been teaching a course called Einstein for Everyone for a long time. The goal of the course is very simple. Anyone who is in an academic environment at some point hears that what Einstein did somehow changed everything. You might run into someone who has some completely crazy idea, and you know the idea is completely crazy, and they tell you, “Blah blah blah blah blah.” And you want to say to them, “Oh, that’s just blah blah blah blah blah. That’s just complete nonsense.” But then they say, “And we know that this is the case because of what Einstein did.” Now you're stuck. You don’t know any physics. You say, “Well, yes, that’s right. I do know that what Einstein did overturned everything. Maybe this guy is right.” My goal in this course is simply to equip people who don’t have the physics with enough of an understanding of what’s happening in modern physics in the wake of Einstein, so they can answer. That’s the goal. I'm not trying to teach physics students physics; I'm trying to teach non-physics students enough of what’s happening in these theories so they have it as a tool that they can use.

The thing that I focus on is a particular way of presenting the ideas. I go through special relativity, general relativity, quantum mechanics, cosmology, black holes, a bunch of material like that. The main thing I'm trying to get across to them is the logical structure that’s involved in each. It’s very easy when you do this kind of introductory instruction to non-specialists to make the theories simply a parade of grotesqueries. First there’s this weird thing that happens, and that weird thing that happens. You know, you can never go faster than the speed of light. Who would have imagined that? It just seems crazy. You try and accelerate yourself through the speed of light; you can’t do it.

But Einstein was a genius and he figured it out. That’s very dismissive. Because the person is being told, “We know things you don’t know and can never know. It’s beyond your understanding. Just believe us.” I wanted that not to happen. So I put a lot of effort into making clear how the logic of it fits together. So someone who goes through the course will understand why it makes perfect sense that you cannot accelerate something through the speed of light. And one of the teasers I had early on is this: it makes perfect sense that for some observers, if an object is moving faster than the speed of light, it’s going backwards in time. Now that seems like a completely crazy and strange idea, right? But by the time you've gotten through enough weeks of the material, it’s completely obvious and it’s not even surprising. You will say, “Yes, obviously. Yes.” So, who am I writing for?

Initially I was writing only for the students in the class. But then I thought, why limit the material that I write just for those students? Professors tend to put their material in password protected websites, and I didn't see a point in doing that. So I just put everything up on my public website. It’s very searchable. Google finds it. And so huge numbers of people are downloading the material and reading it. And I get a steady stream of inquiries about the material from people who just want to figure out what’s going on. I do my best to answer. There’s only so much you can do in an email exchange. But I've been very satisfied with that. I'm getting hundreds of thousands of hits every year.

Zierler:

And it sounds like one of the motivations, or at least this is what you're seeing happening, is that on a very basic level, you're democratizing knowledge. You're showing that these are things that can be understandable to non-specialists. And that’s a very powerful thing for people to understand about themselves.

Norton:

Yes. I never thought of it as democratizing knowledge, but that’s exactly right. That’s exactly what I'm doing. That’s very nice! [laugh] That could be a little epigram.

Zierler:

[laugh] Well, John, on that note, I think I want to ask one last question, a forward-looking question. It’s so clear in looking over your body of scholarship that there’s always that next thing that you're working on, and that you can deduce a larger narrative in the kinds of things that you've been interested in over the years. So I want to ask, looking ahead for the rest of your career, what are some of the big things that you want to accomplish?

Norton:

Well, my major project at the moment is the foundations of inductive inference, and I've written one book that’s up on my website, and I've got a second book that’s largely written. I want to close off that project. It’s really a different way of understanding how inductive inference works. This is the relationship between evidence and theory. Why is it that we think that this piece of evidence, cosmic background radiation, is strong evidence for Big Bang cosmology? If you're outside of philosophy of science, you might think, “Well, that one problem of how evidence supports theory must have been dealt with fully by philosophers of science.” No, it hasn’t. It’s a mess when you see it as I do from the inside. At the moment, the dominant view is that it’s all probabilities, and probabilistic analysis will give you everything. It became very clear to me that probabilistic analysis is not general enough to be able to handle the problem. In appropriately narrow circumstances, probabilistic analysis is wonderful. But it’s not capable of giving us general results about the overall nature of inductive inference. Anyway, I want to get that finished. The next project tends to just fall from the sky, and I never quite know what it’s going to be. I follow the things that interest me. One of the beauties of being in a Department of History and Philosophy of Science is that I'm not grant-driven. I don’t need to have an agenda that will tell me where my next grant is coming from. So when something interests me, I just go and work on it. You've seen my website and my CV. I think it’s fair to say it’s all over the place.

Zierler:

Yeah, it is. And I guess to refine my question a little bit, because it’s all over the place, because in a sense your research is all over the place, with the finite amount of time and resources that you have, I guess really the question is, how do you choose what the next thing to work on is?

Norton:

Oh, I just get excited about something, and then I work on it. I mean, I already know what my next project is going to be after this book is finished. I'm very bothered by metaphysicians talking about possibility. They seem to have an idea of possibility, metaphysical possibility they call it, that’s ungrounded in anything that we know of the world. They just seem to think they know things about what can be but now isn’t the case. It’s analogous, I suppose, to the physicists who think they know history of science because they're a physicist. No. Possibility is a physical thing. It’s about our world. And you can only know about it by having experience of the world. It’s an empirical endeavor. Armchair metaphysics cannot supply it. So I've developed an account of possibility and necessity that respects empiricism, and I'm going to write that one up. If anyone takes any notice of it, it’ll get me into a lot of trouble.

Zierler:

[laugh] Well, I hope we don’t have any part in that. [laugh]

Norton:

No, you won’t. The way to get into trouble is to dispute what everyone, or enough of everyone, think is the case. I can do that by myself. I'll just mention one case in that regard. Did you see ever see the dome? Have you ever looked at that?

Zierler:

No.

Norton:

It’s now on the web. You will find it a Wikipedia article on it. They call it “Norton’s dome.” As part of the analysis of causation, I wanted to demonstrate that Newtonian physics was never really deterministic after all. This is known to the experts, especially John Earman, my colleague here, who wrote the book on determinism. But somehow the idea never got out. So I devised the simplest possible demonstration. You’ll find it if you just Google “Norton’s dome.” You'll get to it pretty quickly. It’s a dome shape, with all the usual idealizations. There’s a point particle that sits right at the very apex of the dome and can move frictionless over it. OK? And you set up and solve Newton’s equations for it. Because of the symmetry of the dome, the particle can stay always exactly where it is. That’s one obvious solution. But it turns out that the differential equation that you solve is one of these ones that doesn't have a well-posed initial value problem. So it turns out that there are other solutions in which the particle sets itself into motion spontaneously. It remains at rest and then, at any later time, moves off in some arbitrary direction. The calculation is trivial. It’s two or three lines of calculus. And if you know that Newtonian physics isn’t inherently deterministic, you look at it and you say, “Fancy that.” That’s my attitude. If you have this deep conviction that Newtonian theory is necessarily deterministic, then for you, it is an evil that must be stamped out.

Zierler:

[laugh]

Norton:

So it created a huge fuss. I didn't expect it at all. You can get into a lot of trouble contradicting “what everyone knows,” even if you are doing things that you don’t think are in the slightest troublesome.

Zierler:

[laugh] Well, I'm always excited when I know what my next really good Google search is going to be, so thanks for that. [laugh] John, it has been so fun talking with you. I'm so delighted that we connected. And this is going to be of tremendous value for so many reasons, so I really appreciate the time you spent with me today.

Norton:

Well, thank you very much. I'm not as optimistic as you are about the value of it, but that’s your call. [laugh]